Convincing evidence has ring confiscated by teacher (5)
I believe the answer is:
proof
'convincing evidence' is the definition.
(I've seen this before)
'ring confiscated by teacher' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'confiscated by' indicates putting letters inside.
'teacher' becomes 'prof' (I've seen this in other clues).
'o' going into 'prof' is 'PROOF'.
'has' is the link.
(Other definitions for proof that I've seen before include "Draft copy of printed text" , "Conclusive demonstration" , "Clear evidence" , "mathematical tour de force" , "Test" .)