Voluntary platoon dispersed around island (8)
I believe the answer is:
optional
'voluntary' is the definition.
(I've seen this before)
'platoon dispersed around island' is the wordplay.
'dispersed' indicates anagramming the letters.
'around' is an insertion indicator.
'island' becomes 'i'.
'platoon' is an anagram of 'optonal'.
'optonal' placed around 'i' is 'OPTIONAL'.
(Other definitions for optional that I've seen before include "something extra?" , "By choice, elective" , "Not compulsory, discretionary" , "Such as one can take or leave" , "Open to choice - take your pick" .)