What greens do on bike (7)
I believe the answer is:
recycle
'what greens do' is the definition.
(I've seen this before)
'on bike' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'bike' becomes 'cycle' (synonyms).
're'+'cycle'='RECYCLE'
(Other definitions for recycle that I've seen before include "Make new use of" , "Behave greenly" , "be green" , "attempt to save the world?" , "Convert waste into form to be used again" .)