Boy starts laughing all day (3)
I believe the answer is:
lad
'boy' is the definition.
(informal term for boy)
'starts laughing all day' is the wordplay.
'starts' says to take the initial letters.
The first letters of 'laughing all day' is 'LAD'.
(Other definitions for lad that I've seen before include "Scottish lover" , "Young fellow" , "Boy or youth" , "Stable worker" , "A young chap into ballads" .)