Expire though once healthy (6)
I believe the answer is:
exhale
'expire' is the definition.
(synonyms)
'once healthy' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'healthy' becomes 'hale' (hale can mean fit and healthy).
'ex'+'hale'='EXHALE'
'though' acts as a link.
(Other definitions for exhale that I've seen before include "produce emissions" , "Expel air from the lungs" , "let breath out" , "Give out" , "Emit breath" .)