Unconvincing wit on a committee (9)
I believe the answer is:
cardboard
'unconvincing' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'wit on a committee' is the wordplay.
'wit' becomes 'card' (synonyms).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'a committee' becomes 'board' (board is a kind of committee).
'card'+'board'='CARDBOARD'
Can you help me to learn more?
(Other definitions for cardboard that I've seen before include "stiff material" , "A stiff thick paper" , "without substance" .)