He's presumably on his own (3)
I believe the answer is:
tod
'he's' is the definition.
'presumably on his own' is the wordplay.
I cannot quite see how this works, but
'on' could be 'to' and 'to' is present in the answer.
A single letter 'd' remains 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 tod that I've seen before include "Fox" , "Former English weight for wool - dot it" , "If you're on it, you're on your own" , "One's alone on this" , "Sly person" .)