Voluntary one found in irregular platoon (8)
I believe the answer is:
optional
'voluntary' is the definition.
(thesaurus)
'one found in irregular platoon' is the wordplay.
'one' becomes 'i' (Roman numeral).
'found in' indicates putting letters inside.
'irregular' indicates an anagram.
'platoon' anagrammed gives 'optonal'.
'i' put within 'optonal' is 'OPTIONAL'.
(Other definitions for optional that I've seen before include "Unforced" , "'Elective, not obligatory (8)'" , "'Left to choice, not compulsory (8)'" , "Such as one can take or leave" , "Voluntary - not required" .)