Happy boys in game with yours truly (8)
I believe the answer is:
Here is my best explanation:
'happy' is the definition.
('gladsome' can be similar in meaning to 'happy')
'boys in game with yours truly' is the subsidiary indication.
'boys' becomes 'lads' (lad is a kind of boy**).
'in' is an insertion indicator.
'game' becomes 'go' (go is a kind of game).
'with' says to put letters next to each other.
'yours truly' becomes 'me'.
'lads' put inside 'go' is 'gladso'.