Initially attacking, later is better in defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'initially attacking later is better in' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'attacking later is better in' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Evidence of (or excuse for) being somewhere else at the time" , "Proof of not being at crime site" , "Timely plea?" , "Defence of being elsewhere at time of crime" , "It shows one wasn't at crime scene" .)