Crossword Genius

Sunday best? That's a laugh! (6)

Ross

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?" .)

I've seen this clue in The Telegraph.
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