Pranks fail to entertain boss (9)
I believe the answer is:
mischiefs
'pranks' is the definition.
(I've seen this in another clue)
'fail to entertain boss' is the wordplay.
'fail' becomes 'miss' (missing is a kind of failing).
'to entertain' indicates putting letters inside.
'boss' becomes 'chief' (synonyms).
'miss' enclosing 'chief' is 'MISCHIEFS'.
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