Non-polluting carbon bank (5)
I believe the answer is:
clean
'non-polluting' is the definition.
(I've seen this before)
'carbon bank' is the wordplay.
'carbon' becomes 'C' (chemical symbol).
'bank' becomes 'lean' (bank can mean to lean or tilt).
'c'+'lean'='CLEAN'
(Other definitions for clean that I've seen before include "Clear (of drugs)" , "Innocent" , "far from blue" , "Not dirty" , "thoroughly" .)