Banned peacekeepers start to look terrible (8)
I believe the answer is:
unlawful
'banned' is the definition.
(thesaurus)
'peacekeepers start to look terrible' is the wordplay.
'peacekeepers' becomes 'un'.
'start to' suggests taking the first letters.
'terrible' becomes 'awful' ('awful' can be a synonym of 'terrible').
The initial letter of 'look' is 'l'.
'un'+'l'+'awful'='UNLAWFUL'
(Other definitions for unlawful that I've seen before include "Criminal" , "Illegal" , "Not legally permitted" , "Not permitted by the state, contrary to statutes" , "Illegitimate" .)