Old pupil a bright one nothing escapes (7)
I believe the answer is:
alumnus
'old pupil' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
(Other definitions for alumnus that I've seen before include "Former school pupil" , "Old pupil" , "School or college graduate" , "Former pupil or student" , "(US) college graduate" .)