Great Scot initially in good health (5)
I believe the answer is:
swell
'great' is the definition.
(I know that great can be written as swell)
'scot initially in good health' is the wordplay.
'initially' suggests taking the first letters.
'in good health' becomes 'well' (similar in meaning).
The first letter of 'scot' is 's'.
's'+'well'='SWELL'
(Other definitions for swell that I've seen before include "Fop" , "wonderful" , "Distend" , "mushroom" , "blowing top" .)