Allow Mark to start in shade (7)
I believe the answer is:
scarlet
'shade' is the definition.
(I have seen 'Shade of red ' mean 'scarlet' so perhaps 'shade' could also mean 'scarlet')
'allow mark to start' is the wordplay.
'allow' becomes 'let' (synonyms).
'mark' becomes 'scar' (synonyms).
'to start' is a charade indicator (letters next to each other).
'let' after 'scar' is 'SCARLET'.
'in' acts as a link.
(Other definitions for scarlet that I've seen before include "Cartels (anag.)" , "Murderer" , "Miss O'Hara appearing briefly" , "Cherry-red" , "Vivid red" .)