Engineers bike to use again (7)
I believe the answer is:
recycle
'use again' is the definition.
(to recycle something is to use it again)
'engineers bike' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'bike' becomes 'cycle' (synonyms).
're'+'cycle'='RECYCLE'
'to' is the link.
(Other definitions for recycle that I've seen before include "Use (waste) again" , "Process for further use" , "Process something so it can be used again" , "attempt to save the world?" , "Keep and use again" .)