Hostile lecturer blocks one student (3*)
I believe the answer is:
'hostile' is the definition.
(similar in meaning)
'lecturer blocks one student' is the wordplay.
'lecturer' becomes 'l' (abbreviation for lecturer).
'blocks' indicates putting letters inside (the inside letters form a 'blockage').
'one' becomes 'i' (Roman numeral).
'student' becomes 'l' (as in L-plates for learner drivers).
'l' placed inside 'il' is 'ILL'.
(Other definitions for ill that I've seen before include "Seasick", "Badly - in need of medical care", "'Never speak . . . of the dead'", "Poorly", "Shakespeare: '. . . met by moonlight'".)