Voluntary mobile platoon capturing island (8)
I believe the answer is:
optional
'voluntary' is the definition.
(I've seen this before)
'mobile platoon capturing island' is the wordplay.
'mobile' is an anagram indicator.
'capturing' is an insertion indicator.
'island' becomes 'i'.
'platoon' anagrammed gives 'optonal'.
'optonal' enclosing 'i' is 'OPTIONAL'.
(Other definitions for optional that I've seen before include "something extra?" , "'Elective, not obligatory (8)'" , "Unforced" , "Voluntary - not required" , "By choice, elective" .)