Colour not allowed to leave a mark (4)
I believe the answer is:
scar
'leave a mark' is the definition.
(I've seen this in another clue)
'colour not allowed to' is the wordplay.
'colour' becomes 'scarlet' (scarlet is a kind of colour).
'not' suggests deleting specific letters.
'allowed to' becomes 'let' (to let something happen is to allow it).
'scarlet' with 'let' removed is 'SCAR'.
(Other definitions for scar that I've seen before include "reminder of sword-fight?" , "Mark left by old wound" , "Skin mark; bare crag" , "Disfigure(ment)" , "Bare craggy rock" .)