Overweight boss is a fool (7)
I believe the answer is:
fathead
'a fool' is the definition.
(fathead is a kind of fool)
'overweight boss' is the wordplay.
'overweight' becomes 'fat' (I've seen this before).
'boss' becomes 'head' ('head' can be a synonym of 'boss').
'fat'+'head'='FATHEAD'
'is' acts as a link.
(Other definitions for fathead that I've seen before include "Fool" , "silly ass!" , "Stupid person" , "Dimwit" , "Slang idiot with enlarged cranium?" .)