Opposes further exams plugged by head of school (7)
I believe the answer is:
resists
'opposes further' is the definition.
'resists' can be an answer for 'opposes' (resisting is a kind of opposing). I am unsure of the 'further' bit.
'exams plugged by head of school' is the wordplay.
'exams' becomes 'resits' (I have seen 'second exams' mean 'resits' so perhaps 'exams' could also mean 'resits').
'plugged by' indicates putting letters inside (plug can mean to block up a gap).
'head of' says to take the initial letters.
The initial letter of 'school' is 's'.
'resits' placed around 's' is 'RESISTS'.
Can you help me to learn more?
(Other definitions for resists that I've seen before include "Defends" , "Struggles against" , "Is hostile to" , "Tries to stop" , "Opposes, puts up fight" .)