Lightweight timber (5)
'lightweight timber' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for balsa that I've seen before include "A slab (anag.)" , "Strong lightweight wood" , "lightweight boat-builder" , "Wood used by model makers" , "Lightweight timber" .)
(Other definitions for scrub that I've seen before include "insignificant type" , "Curbs (anag.)" , "Do away with" , "Clean thoroughly - cancel" , "Clean by rubbing really hard" .)