Solid wood with marble top (4)
I believe the answer is:
firm
'solid' is the definition.
('firm' can be a synonym of 'solid')
'wood with marble top' is the wordplay.
'wood' becomes 'fir' (fir is a kind of wood).
'with' is a charade indicator (letters next to each other).
'top' indicates taking the first letters.
The initial letter of 'marble' is 'm'.
'fir'+'m'='FIRM'
(Other definitions for firm that I've seen before include "Resolute kind of business" , "unlikely to collapse" , "Compact - company" , "Resolute; company" , "Solid - business" .)