Oh, him! (3)
I believe the answer is:
boy
'him' is the definition.
'oh' is the wordplay.
I cannot quite understand how this works, but
'oh' could be 'o' and 'o' is found in the answer.
The remaining letters 'by' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for boy that I've seen before include "Young male" , "Son" , "Youthful male" , "Lad" , "Youth" .)