Air tension (6)
I believe the answer is:
strain
I believe this clue is a double definition.
'air' is the first definition.
(I know that air can be written as strain)
'tension' is the second definition.
(synonyms)
(Other definitions for strain that I've seen before include "In arts (anag)" , "Tension on trains" , "Tension - stock" , "Strive; ancestry" , "Family; effort" .)