Inhalation on filling vessel (6)
I believe the answer is:
breath
'inhalation' is the definition.
(I've seen this in another clue)
'on filling vessel' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'filling' indicates putting letters inside.
'vessel' becomes 'bath' (bath is a kind of vessel).
're' put within 'bath' is 'BREATH'.
(Other definitions for breath that I've seen before include "Inhale and exhale air" , "Intake of air" , "Air in an out of the lungs" , "Bertha (anag.)" , "Air received and expelled" .)