Dishonest behaviour in joke shop, say? (5,8)
I believe the answer is:
funny business
'dishonest behaviour' is the definition.
(funny business means mischief or dishonesty)
'in joke shop say?' is the wordplay.
I cannot really see how this works, but
'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for funny business that I've seen before include "Deceptive or disobedient behaviour (of a clown?)" , "Jiggery-pokery" , "Hanky-panky" , "Shenanigans" , "company of comedians?" .)