Son not ill? Excellent! (5)
I believe the answer is:
swell
'excellent' is the definition.
(I've seen this before)
'son not ill?' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'not ill?' becomes 'well' ('well' is the opposite to 'ill').
's'+'well'='SWELL'
(Other definitions for swell that I've seen before include "Inflate or enlarge, like posh person" , "Balloon - dandy" , "Enlarge; sea movement" , "Heave of the sea's surface" , "Splendid - organ stop" .)