Headgear secured by male volunteers finally on island (9)
I believe the answer is:
manhattan
'island' is the definition.
(I've seen this before)
'headgear secured by male volunteers finally on' is the wordplay.
'headgear' becomes 'hat' (hat is a kind of headgear).
'secured by' is an insertion indicator (I've seen this in other clues).
'male' becomes 'man' (man is a kind of male).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'finally' indicates one should take the final letters.
The final letter of 'on' is 'n'.
'man'+'ta'='manta'
'hat' going inside 'manta' is 'manhatta'.
'manhatta'+'n'='MANHATTAN'
(Other definitions for manhattan that I've seen before include "New York City borough" , "area of New York City" , "Vermouth and whisky cocktail - New York borough" , "One of the five boroughs of New York City" , "Cocktail consisting of whisky, vermouth and bitters" .)