Student finally visiting teacher in jail (4)
I believe the answer is:
stir
'jail' is the definition.
(stir can informally mean jail)
'student finally visiting teacher' is the wordplay.
'finally' suggests the final letters.
'visiting' is an insertion indicator.
'teacher' becomes 'sir' (way of addressing a male teacher).
The final letter of 'student' is 't'.
't' going inside 'sir' is 'STIR'.
'in' is the link.
(Other definitions for stir that I've seen before include "Prison (slang) - disturbance" , "Agitate; wake from sleep" , "Mix or agitate" , "Wake; disturbance" , "Detention centre" .)