Observance that's hackneyed? Not at first (4)
I believe the answer is:
rite
'observance' is the definition.
(thesaurus)
'hackneyed? not at first' is the wordplay.
'hackneyed?' becomes 'trite' ('trite' can be a synonym of 'hackneyed').
'not at first' means to remove the first letter.
'trite' with its first letter taken off is 'RITE'.
'that's' acts as a link.
(Other definitions for rite that I've seen before include "Religious ritual" , "(Words for) ceremony" , "Established religious ceremony" , "Customary observance" , "something like mass" .)