Mark of wound allowed to be red (7)
I believe the answer is:
Here is my best explanation:
'be red' is the definition.
('scarlet' can be a synonym of 'red')
'mark of wound allowed' is the subsidiary indication.
'mark of wound' becomes 'scar'.
'allowed' becomes 'let'.
'to' is the link.
(Other definitions for scarlet that I've seen before include "Clarets (anag.)", "Cartels (anag.)", "Let cars be this colour", "Bright red of clarets", "Vivid red", "Shade of red", "Crimson", "The colour of clarets", "Cherry-red" and "Red colour of clarets".)