Excellent principal admits English students (7)
I believe the answer is:
stellar
'excellent' is the definition.
(I've seen this before)
'principal admits english students' is the wordplay.
'principal' becomes 'star' (synonyms).
'admits' indicates putting letters inside (to admit is to allow in).
'english' becomes 'E' (abbreviation as in OED).
'students' becomes 'LL' (two abbreviations for learner).
'e'+'ll'='ell'
'star' going around 'ell' is 'STELLAR'.
(Other definitions for stellar that I've seen before include "Top-notch" , "Starry" , "Like highlights" , "celebrity" , "Astral" .)