Look after learner in den (4)
I believe the answer is:
lair
'den' is the definition.
(synonyms)
'look after learner' is the wordplay.
'look' becomes 'air' (something might have a certain air or look).
'after' means one lot of letters go next to another.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'air' put after 'l' is 'LAIR'.
'in' is the link.
(Other definitions for lair that I've seen before include "Habitat of a wild animal" , "Place of seclusion (informal)" , "Private place of seclusion" , "mire where he lives" , "Rail around hiding-place" .)