Dull part of a wooden seat (5)
I believe the answer is:
dense
'dull' is the definition.
(synonyms)
'part of a wooden seat' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'DENSE' is hidden in the letters of 'wooden seat'.
(Other definitions for dense that I've seen before include "Thick, compact or crowded" , "Thick and tightly-packed" , "Close knit" , "Obtuse - impenetrable" , "Not very bright" .)