Close business as a joke (6)
I believe the answer is:
windup
'close' is the definition.
(thesaurus)
'business as a joke' is the wordplay.
I cannot quite see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for windup that I've seen before include "Finally close business or tighten watch spring (4, 2)" , "Tease or bring to an end" , "Tighten watch spring or bring to an end" .)