Pine box cut beginning to weaken arm (7)
I believe the answer is:
longbow
'arm' is the definition.
(longbow is a kind of arm)
'pine box cut beginning to weaken' is the wordplay.
'pine' becomes 'long' ('long' can be a synonym of 'pine'**).
'cut' means to remove the last letter.
'beginning to' says to take the initial letters.
The first letter of 'weaken' is 'w'.
'box' with its final letter removed is 'bo'.
'long'+'bo'+'w'='LONGBOW'
(Other definitions for longbow that I've seen before include "Weapon shooting arrows" , "Archer's weapon" , "Mediaeval weapon" , "Ancient weapon" , "Powerful wooden weapon used against the French at Agincourt" .)