Where to lie in a level-headed manner! (4)
I believe the answer is:
lair
'where to lie' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'a level-headed manner' is the wordplay.
'a level-headed' becomes 'l' (this might be a standard abbreviation which I don't know about).
'manner' becomes 'air' (air is a kind of manner).
'l'+'air'='LAIR'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for lair that I've seen before include "mire where he lives" , "Flashy man" , "A nautical hail" , "Wild animal's retreat" , "Place of seclusion (informal)" .)