Would be sound timber (4)
I believe the answer is:
wood
'timber' is the definition.
(I've seen this before)
'would be sound' is the wordplay.
'be sound' shows a homophone (sound like).
'would' is a homophone of 'WOOD'.
(Other definitions for wood that I've seen before include "Copse" , "deal?" , "Small forest; golf club" , "Golf club - tree material" , "Timber" .)