Mark part of test (7)
I believe the answer is:
inkblot
'test' is the definition.
(I've seen this before)
'mark part' is the wordplay.
I cannot quite understand how this works, but
'mark' could be 'blot' (I have seen 'bad mark' mean 'blot' so perhaps 'mark' could also mean 'blot') and 'blot' is found in the answer.
'part' could be 'in' (synonyms) and 'in' is present in the answer.
A single letter 'k' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for inkblot that I've seen before include "Smudge" , "Result of pen leak" , "meaningful stain?" , "Stain of various shapes and colours used in the Rorschach test" , "Something used in Rorschach test" .)