Boom-box (4)
I believe the answer is:
spar
I believe this is a double definition.
'boom' is the first definition.
(I've seen this before)
'box' is the second definition.
(spar can mean to box)
(Other definitions for spar that I've seen before include "Box; various minerals" , "Wooden pole -- box" , "Box gently; main beam of aircraft wing" , "Practice boxing with wooden beam" , "Act as boxer" .)