New alliance, critical, not half, opposing ministers? (12)
I believe the answer is:
anticlerical
'opposing ministers?' is the definition.
'new alliance critical not half' is the wordplay.
'new' indicates anagramming the letters.
'not half' suggests halving the letters.
'critical' with half the letters taken is 'crit'.
'alliance'+'crit'='alliancecrit'
'alliancecrit' is an anagram of 'ANTICLERICAL'.
(Other definitions for anticlerical that I've seen before include "Resentful of priests" , "Opposed to the power of the clergy" .)