Tree beginning to mature fast (4)
I believe the answer is:
'fast' is the definition.
('firm' can be a synonym of 'fast')
'tree beginning to mature' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree**).
'beginning to' suggests taking the first letters.
The initial letter of 'mature' is 'm'.
(Other definitions for firm that I've seen before include "Resolute; company" , "Tight; company" , "Unyielding business concern?" , "Resolute kind of business" , "securely rooted" .)