Sunday best? That's a laugh! (6)
I believe the answer is:
scream
'that's a laugh' is the definition.
(scream is a kind of laugh)
'sunday best?' is the wordplay.
'sunday' becomes 's'.
'best?' becomes 'cream' (I've seen this before).
's'+'cream'='SCREAM'
(Other definitions for scream that I've seen before include "Extremely funny entertainer" , "Yell for creams" , "Hilarious character" , "A shrill yell as from fright" , "noise?" .)