Express discontent about timber used for furniture (6)
I believe the answer is:
repine
'express discontent' is the definition.
(repining is a kind of expressing discontent)
'about timber used for furniture' is the wordplay.
'about' becomes 're' (regarding).
'timber used for furniture' becomes 'pine'.
're'+'pine'='REPINE'
(Other definitions for repine that I've seen before include "Express discontent, fret" , "Fret, be discontented" , "Fret, about a tree perhaps?" , "Plain" , "Be in a state of anxiety" .)