Blaspheme after Sunday display (5)
I believe the answer is:
swear
'blaspheme' is the definition.
('swear' can be a synonym of 'blaspheme')
'after sunday display' is the wordplay.
'after' means one lot of letters go next to another.
'sunday' becomes 's'.
'display' becomes 'wear' (I've seen this before).
's' put next to 'wear' is 'SWEAR'.
(Other definitions for swear that I've seen before include "Express annoyance" , "Promise on oath" , "Use indecent or blasphemous language" , "not to mince one's words" , "Affirm" .)