Retreats in total air setback (5)
I believe the answer is:
lairs
'retreats' is the definition.
(I've seen this in another clue)
'in total air setback' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LAIRS' is hidden in the letters of 'total air setback'.
(Other definitions for lairs that I've seen before include "Scottish landowners" , "Places to hide" , "Resting places of wild animals" , "Animals' dens" , "forms etc" .)