Red mark permitted (7)
I believe the answer is:
scarlet
'red' is the definition.
(I know this)
'mark permitted' is the wordplay.
'mark' becomes 'scar' (synonyms).
'permitted' becomes 'let' (to let something happen is to permit it).
'scar'+'let'='SCARLET'
(Other definitions for scarlet that I've seen before include "Brilliant red colour" , "Crimson" , "immoral" , "Vivid red" , "Clarets (anag.)" .)