Mostly red bike to be green? (7)
I believe the answer is:
recycle
'be green?' is the definition.
(recycling is a way of being green)
'mostly red bike' is the wordplay.
'mostly' means to remove the last letter.
'bike' becomes 'cycle' (synonyms).
'red' with its last letter taken away is 're'.
're'+'cycle'='RECYCLE'
'to' acts as a link.
(Other definitions for recycle that I've seen before include "salvage for further use" , "Process for re-use" , "Use (rubbish) again" , "attempt to save the world?" , "Keep and use again" .)