No longer fit to breathe (6)
I believe the answer is:
exhale
'breathe' is the definition.
(I know that exhale is a kind of breathe)
'no longer fit' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'fit' becomes 'hale' (both can mean healthy).
'ex'+'hale'='EXHALE'
'to' is the link.
(Other definitions for exhale that I've seen before include "Expel air from the lungs" , "Puff perhaps" , "let breath out" , "Puff away" , "Breathe out" .)