Mark someone's card somewhat (4)
I believe the answer is:
scar
'mark' is the definition.
('scar' can be a synonym of 'mark')
'someone's card somewhat' is the subsidiary indication.
'somewhat' indicates a hidden word.
'scar' is hidden within 'someones card'.
(Other definitions for scar that I've seen before include "A bare precipice" , "Evidence of an old cut?" , "Evidence of an old injury" , "Visible mark of healing" , "Craggy rock formation" .)