Hammering head of rivet exposed (4)
I believe the answer is:
rout
'hammering' is the definition.
(both can mean a decisive defeat)
'head of rivet exposed' is the wordplay.
'head of' suggests taking the first letters.
'exposed' becomes 'out' (to out someone is to expose them).
The initial letter of 'rivet' is 'r'.
'r'+'out'='ROUT'
(Other definitions for rout that I've seen before include "Defeat disastrously" , "Inflict heavy defeat on" , "Runaway defeat" , "Vanquish; thrash" , "snore" .)