Look after lion's first den (4)
I believe the answer is:
lair
'den' is the definition.
(synonyms)
'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' put 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" .)