A boy had been slow (5)
I believe the answer is:
abate
'a boy had been slow' is the definition.
I can't judge whether this definition defines the answer.
'a boy had' is the wordplay.
'a boy' becomes 'ab'.
'had' becomes 'ate' (e.g. I had an ice cream).
'ab'+'ate'='ABATE'
Can you help me to learn more?
(Other definitions for abate that I've seen before include "Decline" , "(Flood) recede" , "Become less strong like storm" , "retreat" , "Lessen in intensity" .)