The bar replaced air (6)
I believe the answer is:
breath
'air' is the definition.
(I know that breath is a type of air)
'the bar replaced' is the wordplay.
'replaced' indicates anagramming the letters.
'the'+'bar'='thebar'
'thebar' with letters rearranged gives 'BREATH'.
(Other definitions for breath that I've seen before include "Inhale and exhale air" , "It is drawn all the time" , "Intake of air" , "Air in an out of the lungs" , "Bather (anag.)" .)