In favour of turning right (3)
I believe the answer is:
for
'in favour' is the definition.
(as in being for a particular cause)
'of turning right' is the wordplay.
'turning' says the letters should be written backwards.
'right' becomes 'r' (common abbreviation).
'of' back-to-front is 'fo'.
'fo'+'r'='FOR'
(Other definitions for for that I've seen before include "Approving" , "Pro" , "In favour of; since" , "and 16 and 20: Notwithstanding" , "On behalf of" .)