Boy holding end of the leash (4)
I believe the answer is:
lead
'leash' is the definition.
(I know that leash can be written as lead)
'boy holding end of the' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'holding' means one lot of letters goes inside another.
'end of' suggests the final letters.
The final letter of 'the' is 'e'.
'lad' enclosing 'e' is 'LEAD'.
(Other definitions for lead that I've seen before include "first guitarist" , "Metal; show the way" , "Take a position in front" , "Guide, conduct" , "First news item" .)