Bigwig smiles first, feeling good (5)
I believe the answer is:
swell
'good' is the definition.
(similar in meaning)
'bigwig smiles first feeling' is the wordplay.
I cannot really understand how this works, but
'smiles first' could be 's' (1st letter of 'smiles') and 's' is found within the answer.
The remaining letters 'well' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for swell that I've seen before include "Bulge; a nob" , "First class" , "organ feature" , "Enlarge; sea movement" , "mushroom" .)