Respiration in the bar varied (6)
I believe the answer is:
breath
'respiration' is the definition.
(I've seen this before)
'the bar varied' is the wordplay.
'varied' indicates anagramming the letters.
'the'+'bar'='thebar'
'thebar' with letters rearranged gives 'BREATH'.
'in' is the link.
(Other definitions for breath that I've seen before include "Bather (anag.)" , "An inhalation and exhalation" , "Air drawn in and expelled out" , "from which inspiration may be drawn" , "It is drawn all the time" .)