Secure line during party (5)
I believe the answer is:
clamp
'secure' is the definition.
(clamping is a kind of securing)
'line during party' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'during' is an insertion indicator.
'party' becomes 'camp' (both can mean a group of people).
'l' placed within 'camp' is 'CLAMP'.
(Other definitions for clamp that I've seen before include "Of car, render immobile for parking offence" , "Firmly hold" , "Immobilise (illegally parked car)" , "Grip firmly" , "pile of turf" .)