Fathead exploits bonds (5)
I believe the answer is:
fuses
'bonds' is the definition.
(I've seen this before)
'fathead exploits' is the wordplay.
'fathead' becomes 'f' (head letter of 'fat').
'exploits' becomes 'uses' (use is a kind of exploit).
'f'+'uses'='FUSES'
(Other definitions for fuses that I've seen before include "Blends together as with heat" , "Blow one of these and lose your temper" , "bonds" , "They interrupt the electric current when they're overloaded" , "Electrical safety devices" .)