He's right beside the tree (6)
I believe the answer is:
oliver
'he's' is the definition.
'right beside the tree' is the wordplay.
I cannot quite see how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found within the answer.
'tree' could be 'olive' (olive is a kind of tree) and 'olive' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oliver that I've seen before include "First name of Dickens boy Twist" , "Fictional boy who famously asked for more" , "Hardy, maybe" , "Roland companion; Bath biscuit" , "Dickens-based musical" .)