Many a student joke is a bore (4)
I believe the answer is:
drag
'a bore' is the definition.
(I've seen this before)
'many a student joke' is the wordplay.
'many' becomes 'D' (Roman numeral for 500).
'a student joke' becomes 'rag' (I can't justify this - if you can you should believe this answer much more).
'd'+'rag'='DRAG'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for drag that I've seen before include "What female impersonators assume" , "A bore (colloq.); lug" , "Tedious experience" , "Resistance to motion" , "Hunt without foxes" .)