Den left to dry out (4)
I believe the answer is:
lair
'den' is the definition.
('lair' can be a synonym of 'den')
'left to dry out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'to dry out' becomes 'air' (airing is a kind of drying out).
'l'+'air'='LAIR'
(Other definitions for lair that I've seen before include "office?" , "Secret base" , "Wild animal's place of refuge" , "enclosure" , "Den of a wild animal" .)