A genius being skinned by mug (8)
I believe the answer is:
einstein
'a genius' is the definition.
(I've seen this before)
'being skinned by mug' is the wordplay.
'skinned' says to take the centre.
'by' means one lot of letters go next to another.
'mug' becomes 'stein' (stein is a kind of mug**).
The centre of 'being' is 'ein'.
'ein'+'stein'='EINSTEIN'
(Other definitions for einstein that I've seen before include "Brilliant scientist" , "Genius; physicist" , "Scientific genius" , "Mathematical genius of nineties" , "He contributed the theory of relativity" .)