Mark heads back (4)
I believe the answer is:
spot
'mark' is the definition.
(I know that spot is a type of mark)
'heads back' is the wordplay.
'heads' becomes 'tops'.
'back' says the letters should be written backwards.
'tops' backwards is 'SPOT'.
(Other definitions for spot that I've seen before include "See; location" , "Informally, catch sight of" , "jam > stain" , "See 12" , "Fix" .)