Opposing military commander isn’t bad (7)
I believe the answer is:
against
'opposing' is the definition.
(I've seen this before)
'military commander isn't bad' is the wordplay.
'military commander' becomes 'aga' (historical commander in some Muslim countries).
'bad' is an anagram indicator.
'isnt' anagrammed gives 'inst'.
'aga'+'inst'='AGAINST'
(Other definitions for against that I've seen before include "In opposition to" , "In disagreement with" , "Disapproving of" , "close by" , "Con" .)