Mark, the stand-in deprived of a part (5)
I believe the answer is:
stain
'mark' is the definition.
(I know that mark can be written as stain)
'stand-in deprived of a part' is the wordplay.
'stand in deprived of' becomes 'st' (I am not sure about this - if you are sure you should believe this answer much more).
'part' becomes 'in part' (synonyms).
'st'+'a'+'in'='STAIN'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for stain that I've seen before include "Stigma" , "Smear" , "Mark with colour, dirt" , "Black mark for saint" , "Spot - shame" .)