Cut top off non-polluting tree (9)
I believe the answer is:
evergreen
'tree' is the definition.
(I've seen this before)
'cut top off non-polluting' is the wordplay.
'cut' becomes 'sever' (to sever is to cut or split).
'top off' means to remove the first letter.
'non-polluting' becomes 'green' (I can't explain this - if you can you should give a lot more credence to this answer).
'sever' with its initial letter taken away is 'ever'.
'ever'+'green'='EVERGREEN'
Can you help me to learn more?
(Other definitions for evergreen that I've seen before include "Fresh and vital" , "Continually fresh" , "Ivy, perhaps" , "Having enduring popularity" , "Laurel for one" .)