Solid wood with marble top (4)
I believe the answer is:
Here is my best explanation:
'solid' is the definition.
('firm' can be a synonym of 'solid')
'wood with marble top' is the subsidiary indication.
'wood' becomes 'fir' (fir is a kind of wood).
'with' means one lot of letters go next to another.
'top' suggests taking the first letters.
The initial letter of 'marble' is 'm'.
(Other definitions for firm that I've seen before include "Steady kind of business", "Unyielding; company", "Stable; business", "Company; unyielding", "Well-established - business", "Resolute kind of business", "Steady commercial organisation?", "Business company", "Solid; secure" and "Steadfast".)