On island, young animal hasn't left foot (4)
I believe the answer is:
iamb
'foot' is the definition.
(type of metrical foot in poetry)
'on island young animal hasn't left' is the wordplay.
'on' is a charade indicator (letters next to each other).
'island' becomes 'i' (geographical abbreviation).
'young animal' becomes 'lamb' (young sheep).
'hasn't' suggests deleting specific letters.
'left' becomes 'l' (common abbreviation).
'lamb' with 'l' taken out is 'amb'.
'i' put next to 'amb' is 'IAMB'.
(Other definitions for iamb that I've seen before include "Short-long metrical foot" , "Line of verse" , "Metrical foot of one short, one long syllables" , "Part of a pentameter?" , "Short-long metric foot" .)