Happy with a grand fellow (4)
I believe the answer is:
glad
'happy' is the definition.
('glad' can be a synonym of 'happy')
'with a grand fellow' is the wordplay.
'with' says to put letters next to each other.
'a grand' becomes 'g' ('G' can be a synonym of 'grand').
'fellow' becomes 'lad' (I have seen 'Young fellow' mean 'lad' so perhaps 'fellow' could also mean 'lad').
'g'+'lad' is 'GLAD'.
(Other definitions for glad that I've seen before include "Informal flower < -girl" , "Contented" , "Pleased, delighted" , "Happy, pleased" , "These rags are dressy clothes - happy?" .)