Struggle with package's first buckle (4)
I believe the answer is:
warp
'buckle' is the definition.
(I know that buckle can be written as warp)
'struggle with package's first' is the wordplay.
'struggle' becomes 'war' (warring is a kind of struggling).
'with' says to put letters next to each other.
'package's first' becomes 'p' (1st letter of 'package').
'war'+'p'='WARP'
(Other definitions for warp that I've seen before include "Crane fly larva" , "Abnormality in character" , "Distort, ship's rope" , "Pervert - lengthways threads" , "Of wood, twist under heat" .)