A superior holy person before becoming stern (7)
I believe the answer is:
austere
'stern' is the definition.
('austere' can be a synonym of 'stern')
'a superior holy person before' is the wordplay.
'superior' becomes 'u' (I've seen this in other clues).
'holy person' becomes 'st' (I've seen this in other clues).
'before' becomes 'ere' (usually literary).
'a'+'u'+'st'+'ere'='AUSTERE'
'becoming' is the link.
(Other definitions for austere that I've seen before include "Abstemious and grave" , "Strict in appearance" , "Abstemious, strict" , "Harsh" , "Grim - USA tree (anag)" .)