Paint to be treated in a church, if absolutely necessary (2,1,5)
I believe the answer is:
at a pinch
'if absolutely necessary' is the definition.
(I've seen this before)
'paint to be treated in a church' is the wordplay.
'to be treated' indicates anagramming the letters.
'in' indicates putting letters inside.
'church' becomes 'ch' (common abbreviation).
'paint' is an anagram of 'tapin'.
'a'+'ch'='ach'
'tapin' put within 'ach' is 'AT A PINCH'.
(Other definitions for at a pinch that I've seen before include "In time of difficulty or necessity - nip it" , "If absolutely necessary" , "If really necessary" , "under pressure" , "in a crisis?" .)