Mark found in Christmas card (4)
I believe the answer is:
scar
'mark' is the definition.
('scar' can be a synonym of 'mark')
'in christmas card' is the wordplay.
'in' indicates a hidden word.
'SCAR' is hidden in the letters of 'christmas card'.
'found' acts as a link.
(Other definitions for scar that I've seen before include "Effect of unpleasant experience" , "Mark left on healed tissue" , "Craggy rock formation" , "Evidence of an old injury" , "reef" .)