Slips into (4)
I believe the answer is:
dons
'slips into' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dons that I've seen before include "Assumes" , "Academics" , "Puts on -- lecturers" , "University teachers - Mafia bosses" , "British university teachers and Spanish noblemen" .)