Box available for use, no end (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'available for use no end' is the wordplay.
'available for use' becomes 'spare' (something going spare is available for use).
'no end' means to remove the last letter.
'spare' with its last letter taken away is 'SPAR'.
(Other definitions for spar that I've seen before include "dispute" , "Practice boxing with wooden beam" , "Do up (bird for oven)" , "Crystal - box" , "Box (gently) - fight (verbally)" .)