Tumbler indeed on top of a mug (7)
I believe the answer is:
acrobat
'tumbler' is the definition.
(synonyms)
'indeed on top of a mug' is the wordplay.
I cannot really understand how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is present in the answer.
'of' could be 'o' and 'o' is present in the answer.
'a' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for acrobat that I've seen before include "Circus gymnast" , "Gymnastic performer" , "Circus star" , "Tumbler" , "Performing gymnast" .)