Air pressure (6)
I believe the answer is:
strain
This clue is a double definition.
'air' is the first definition.
(I know that air can be written as strain)
'pressure' is the second definition.
(I've seen this before)
(Other definitions for strain that I've seen before include "song" , "In arts (anag)" , "Tension - stock" , "Pressure - filter" , "Struggle" .)