Boy happy heading off (3)
I believe the answer is:
lad
'boy' is the definition.
(lad is a kind of boy)
'happy heading off' is the wordplay.
'happy' becomes 'glad' ('glad' can be similar in meaning to 'happy').
'heading off' means to remove the first letter.
'glad' with its first letter removed is 'LAD'.
(Other definitions for lad that I've seen before include "Boy, young man" , "(Housman's Shropshire) boy" , "Shaver" , "Boy or youth" , "Youngster" .)