Wheel about to go back (3)
I believe the answer is:
cog
'wheel' is the definition.
(wheel with teeth in a machine)
'about to go back' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to' says to put letters next to each other.
'back' says the letters should be written backwards.
'go' written backwards gives 'og'.
'c'+'og'='COG'
(Other definitions for cog that I've seen before include "Projection on a toothed wheel" , "Tooth on gearwheel" , "Little gearwheel" , "Minor player" , "Machine component" .)