Turn back on green (6)
I believe the answer is:
revert
'turn back' is the definition.
(I know that turn back can be written as revert)
'on green' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'green' becomes 'vert' (I've seen this before).
're'+'vert'='REVERT'
(Other definitions for revert that I've seen before include "Return to (earlier state)" , "Return to former state (6)" , "Go back to former condition" , "Go back to a former practice" , "Go back (to topic)" .)