Oddball master is in good health (6)
I believe the answer is:
misfit
'oddball' is the definition.
(I've seen this before)
'master is in good health' is the wordplay.
'master' becomes 'm'.
'in good health' becomes 'fit' (I've seen this before).
'm'+'is'+'fit'='MISFIT'
(Other definitions for misfit that I've seen before include "not a suitable one" , "Person not well suited to life or environment" , "Person who cannot integrate into society" , "An unsuitable thing" , "Social outcast" .)