Girl lasts in sumo wrestling among island people (6)
I believe the answer is:
imogen
'girl' is the definition.
(I've seen this before)
'lasts in sumo wrestling among island people' is the wordplay.
'lasts in' suggests the final letters (I've seen 'last in' mean this).
'among' is an insertion indicator.
'island' becomes 'i'.
'people' becomes 'men' (man is a kind of person).
The final letters of 'sumo wrestling' are 'og'.
'i'+'men'='imen'
'og' inserted inside 'imen' is 'IMOGEN'.
(Other definitions for imogen that I've seen before include "Princess in "Cymbeline"" , "Daughter of Cymbeline in Shakespeare's play" , "identity of Cymbeline's daughter" , "Character 8 [OF SHAKESPEARE]" , "child of a king" .)