Crack a potty (4)

I believe the answer is:

gaga

'potty' is the definition.
(thesaurus)

'crack a' is the wordplay.
'crack' becomes 'gag' (both can mean a joke).
'gag'+'a'='GAGA'

(Other definitions for gaga that I've seen before include "bananas" , "Senile or crazy in repetitive way" , "person who is senile?" , "fatuous?" , "Very fond of" .)

I've seen this clue in The Times.

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