Permissive one in platoon out of order (8)
I believe the answer is:
optional
'permissive' is the definition.
('optional' can be similar in meaning to 'permissive')
'one in platoon out of order' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'out of order' indicates anagramming the letters.
'platoon' with letters rearranged gives 'optonal'.
'i' going inside 'optonal' is 'OPTIONAL'.
(Other definitions for optional that I've seen before include "something extra?" , "Platoon I may leave to choice" , "Too plain (anag.)" , "Not mandatory" , "Open to choice - take your pick" .)