Excuse groom taking time before church (8)
I believe the answer is:
pretence
'excuse' is the definition.
(thesaurus)
'groom taking time before church' is the wordplay.
'groom' becomes 'preen' (a bird grooms itself by preening feathers).
'taking' is an insertion indicator.
'time' becomes 't' (abbreviation).
'before' says to put letters next to each other.
'church' becomes 'CE' (Church of England).
'preen' going around 't' is 'preten'.
'preten'+'ce'='PRETENCE'
(Other definitions for pretence that I've seen before include "Act of make-believe" , "Deceit, dissimulation" , "Make-believe, perhaps a phoney claim to status (8)" , "False show hiding reality" , "it can't be true!" .)