Thick part of a wooden seat (5)
I believe the answer is:
dense
'thick' 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 within 'wooden seat'.
(Other definitions for dense that I've seen before include "Close-knit" , "Thick, compact or crowded" , "Compressed - obtuse" , "Closely packed" , "Slow-witted" .)