Breathe out when no longer well? (6)
I believe the answer is:
exhale
'breathe out' is the definition.
(synonyms)
'no longer well?' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'well?' becomes 'hale' (I've seen this before).
'ex'+'hale'='EXHALE'
'when' acts as a link.
(Other definitions for exhale that I've seen before include "produce emissions" , "Discharge" , "Puff perhaps" , "Puff away" , "Give out" .)