Mark a bit of headscarf (4)
I believe the answer is:
scar
'mark' is the definition.
('scar' can be a synonym of 'mark')
'a bit of headscarf' is the wordplay.
'a bit of' indicates the answer is hidden within the clue (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'SCAR' is hidden within 'headscarf'.
(Other definitions for scar that I've seen before include "Old wound - bare cliff" , "Mark from a wound left on the skin" , "Mark left by healed wound" , "Permanent mark left by a wound" , "Limestone cliff or outcrop" .)