Originally attacking less, it's better in defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'originally attacking less it's better in' is the wordplay.
'originally' says to take the initial letters.
The initial letters of 'attacking less its better in' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "can offer a valid defence against most charges" , "it may help you to get off" , "Lawyer's weapon, Latin for 'elsewhere'" , "A claim one was elsewhere when crime took place" , "Legal defence - was elsewhere" .)