Marked student’s first, abridged unseen? No (7)
I believe the answer is:
spotted
'marked' is the definition.
(spotting is a kind of marking)
'student's first abridged unseen? no' is the wordplay.
I cannot quite see how this works, but
'abridged' could be 'potted' (potted can mean abridged or condensed) and 'potted' is present in the answer.
'student's first' could be 's' (1st letter of 'student') and 's' is found in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for spotted that I've seen before include "Noticed" , "Saw, caught sight of" , "Observed" , "- Dog; - dick" , "Saw; speckled" .)