Performer, a jazz fan downing mug (7)
I believe the answer is:
acrobat
'performer' is the definition.
(I have seen 'Gymnastic performer ' mean 'acrobat' so perhaps 'performer' could also mean 'acrobat')
'a jazz fan downing mug' is the wordplay.
'jazz fan' becomes 'cat' (I've seen this before).
'downing' means one lot of letters goes inside another (to down food or drink is to swallow it quickly).
'mug' becomes 'rob' (I've seen this in other clues).
'cat' enclosing 'rob' is 'crobat'.
'a'+'crobat'='ACROBAT'
(Other definitions for acrobat that I've seen before include "Performer going head over heels?" , "Performing gymnast" , "Rope-dancer" , "Circus gymnast" , "Circus entertainer" .)