Fixed tree-top in mangrove (4)
I believe the answer is:
firm
'fixed' is the definition.
(I've seen this before)
'tree-top in mangrove' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'top in mangrove' becomes 'm' (I am not sure about this - if you are sure you should believe this answer much more).
'fir'+'m'='FIRM'
Can you help me to learn more?
(Other definitions for firm that I've seen before include "Unyielding business concern?" , "'Solid, unyielding (4)'" , "Strong - business" , "Well-established - business" , "Company - solid" .)