Smash defence work (6)
I believe the answer is:
wallop
'smash' is the definition.
(thesaurus)
'defence work' is the wordplay.
'defence' becomes 'wall' (wall is a kind of defence).
'work' becomes 'op' (abbreviation for opus).
'wall'+'op'='WALLOP'
(Other definitions for wallop that I've seen before include "Beer - hit hard" , "A heavy resounding blow (6)" , "Strike very hard" , "Hit - the booze?" , "big hit" .)