Handle line during strike (4)
I believe the answer is:
hilt
'handle' is the definition.
(hilt is a kind of handle)
'line during strike' is the wordplay.
'line' becomes 'l'.
'during' indicates putting letters inside.
'strike' becomes 'hit' ('hit' can be a synonym of 'strike').
'l' going into 'hit' is 'HILT'.
(Other definitions for hilt that I've seen before include "Handle of a sword or dagger" , "Sword/dagger handle" , "Handle of bladed weapon" , "One may back up to this" .)