He returned under a tree (5)
I believe the answer is:
elmer
'he' is the definition.
'returned under a tree' is the wordplay.
I cannot quite understand how this works, but
'tree' could be 'elm' (elm is a kind of tree) and 'elm' is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for elmer that I've seen before include "Man" , "- - Gantry (book, film)" , "____ Rice, US dramatist" , "name" .)