Look after lion's first den (4)
I believe the answer is:
lair
'den' is the definition.
(I know that den can be written as lair)
'look after lion's first' is the wordplay.
'look' becomes 'air' (something might have a certain air or look).
'after' is a charade indicator (letters next to each other).
'lion's first' becomes 'l' (1st letter of 'lion').
'air' after 'l' is 'LAIR'.
(Other definitions for lair that I've seen before include "Study" , "Rail around hiding-place" , "Home of a beast" , "Wild animal's retreat" , "Animal's resting place" .)