Tree beginning to mature fast (4)
I believe the answer is:
Here is my best explanation:
'fast' is the definition.
('firm' can be a synonym of 'fast')
'tree beginning to mature' is the subsidiary indication.
'tree' becomes 'fir' (fir is a kind of tree).
'beginning to' indicates taking the first letters.
The initial letter of 'mature' is 'm'.
(Other definitions for firm that I've seen before include "Steady commercial organisation?", "Company - solid", "Unyielding business concern?", "Company; unyielding", "Resolute; company", "Steady kind of business", "Tight; company", "'Solid, unyielding (4)'", "Organization" and "Secure - business partnership".)