Old healthy son breathes out (7)
I believe the answer is:
exhales
'breathes out' is the definition.
('exhale' can be a synonym of 'breathe out')
'old healthy son' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'healthy' becomes 'hale' (hale can mean fit and healthy).
'son' becomes 's' (genealogical abbreviation for son).
'ex'+'hale'+'s'='EXHALES'
(Other definitions for exhales that I've seen before include "expires" , "Breathes out" .)