A good thing, only when healthy (4,2,4)
I believe the answer is:
just as well
'a good thing' is the definition.
(I've seen this before)
'only when healthy' is the wordplay.
'only' becomes 'just' (synonyms).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'healthy' becomes 'well' (similar in meaning).
'just'+'as'+'well'='JUST AS WELL'
(Other definitions for just as well that I've seen before include "Fortunate" , "A good thing" , "desirable!" .)