Fat head of cheating partner (6)
I believe the answer is:
chubby
'fat' is the definition.
(I've seen this before)
'head of cheating partner' is the wordplay.
'head of' says to take the initial letters.
'partner' becomes 'hubby' (hubby is a kind of partner).
The initial letter of 'cheating' is 'c'.
'c'+'hubby'='CHUBBY'
(Other definitions for chubby that I've seen before include "with some corporation?" , "Plumb and round" , "Round and pleasantly plump" , "On the plump side" , "Attractively plump" .)