Amice of Gloucester was the 2nd daughter, heiress of her sister, Mabel; Amice married Richard de Clare; she succeeded her sister as Countess of Gloucester in 1217. 
Death of Amice
[Note: C.P. 6 (1926): 503 (sub Hertford) says Amice de Clare, Countess of Hertford "is stated to have died 1 January 1222/5, before which date she appears to have been recognized as Countess of Gloucester." This statement regarding her being Countess of Gloucester appears to be without foundation. In Amice's own charters which have survived, she refers to herself solely as Countess of Clare (i.e., Hertford), and never as Countess of Gloucester (see, for instance, Dugdale et al. Monasticon Anglicanum 6(3) (1830): 1658-1659 (charters of Amice, Countess of Clare, daughter of William Earl of Gloucester); C. Harper-Bill ed. Stoke by Clare Cartulary 1 (Suffolk Charters 4) (1982): 41-48 (charters of Amice, Countess of Clare); Mortimer Charters of St. Bartholomew's Priory (Suffolk Charters 15) (1996): 25-26 (charter of Amice, Countess of Clare). Rather, Dugdale Monasticon Anglicanum 1 (1817): 33 states that Amice's son and heir, Gilbert de Clare, took up the twin earldoms of Gloucester and Hertford in 1217, which occurred during his mother's lifetime. Earl Gilbert certainly had possession of the Gloucester inheritance before 1220/1, when the Pipe Rolls sub Norfolk and Suffolk state that 'Isti habunt quietancias per brevia - Comes de Clara de 131 f etc.' (Reference: Pipe Roll of 5 Henry III cited in C.P. 6 (1926): 503, footnote c). Presumably Amice was excluded from the Gloucester inheritance by the terms of her father's agreement with King Henry II in 1176, by which King Henry's son, John (later King John) was acknowledge as heir to William Earl of Gloucester (as future husband of his daughter, Isabel); in return for this grant, the king agreed to give £100 yearly rental to Earl William's younger daughters, Mabel and Amice (see Lambert Bletchingley: A Parish Hist. 1 (1921): 53-54, 59, footnote 2).] 
Magna Carta Ancestry 2011 2nd ed. Vol. I p. 446-451
Medieval Lands, database online, author Charles Cawley, (Foundation for Medieval Genealogy, 2006-2013), English Earls 1067-1122, Chapter 11, Gloucester: B. EARLS of GLOUCESTER 1122-1225, William FitzRobert...3. Amice Fitzrobert
The Magna Charta Sureties, 1215, 4th ed.
Ancestral Roots of Certain American Colonists Who Came to America before 1700, 7th ed. Author: Weis, Frederick Lewis, Editor: Sheppard Jr., (Walter Lee Publication: Genealogical Publishing Co., Inc., Baltimore, Maryland, 1992)