Teacher last to leave, having finished (4)
I believe the answer is:
done
'finished' is the definition.
('done' can be similar in meaning to 'finished')
'teacher last to leave' is the wordplay.
'teacher' becomes 'don' (don is a kind of teacher**).
'last to' says to take the final letters.
The final letter of 'leave' is 'e'.
'don'+'e'='DONE'
'having' acts as a link.
(Other definitions for done that I've seen before include "Performed" , "This settled" , "Tricked" , "agreed" , "Cooked; carried out" .)