God is pure, men beg to be changed (7,5)
I believe the answer is:
supreme being
'god' is the definition.
('Supreme Being' can be a synonym of 'god')
'is pure men beg to be changed' is the wordplay.
'to be changed' indicates an anagram.
'is'+'pure'+'men'+'beg'='ispuremenbeg'
'ispuremenbeg' is an anagram of 'SUPREME BEING'.
(Other definitions for supreme being that I've seen before include "God" , "object of worship" .)