Den left to ventilate (4)
I believe the answer is:
lair
'den' is the definition.
(I know that den can be written as lair)
'left to ventilate' is the wordplay.
'left' becomes 'l' (common abbreviation).
'to ventilate' becomes 'air' (synonyms).
'l'+'air'='LAIR'
(Other definitions for lair that I've seen before include "animal refuge" , "Wild animal's retreat" , "Supposed large ape, the Bigfoot" , "Earth" , "Hideaway, den" .)