Except if peacekeeping body gets fewer (6)
I believe the answer is:
unless
'except if' is the definition.
(I've seen this before)
'peacekeeping body gets fewer' is the wordplay.
'peacekeeping body' becomes 'un' (I am not sure about this - if you are sure you should believe this answer much more).
'gets' is a charade indicator (letters next to each other).
'fewer' becomes 'less' (associated in meaning).
'un'+'less'='UNLESS'
Can you help me to learn more?
(Other definitions for unless that I've seen before include "Though" , "Conjunction meaning except when" , "bar" , "Except on the condition that" , "Otherwise than if" .)