Concern for tree close to stream (4)
I believe the answer is:
firm
'concern' is the definition.
(I know that firm is a type of concern)
'tree close to stream' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'close to' suggests the final letters (the close/ending of).
The final letter of 'stream' is 'm'.
'fir'+'m'='FIRM'
'for' is the link.
(Other definitions for firm that I've seen before include "'Solid, unyielding (4)'" , "Steady commercial organisation?" , "unlikely to collapse" , "Steady sort of business organisation" , "Business company" .)