Saint to hold one on a tree (9)

'tree' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?

'saint to hold one on a' is the wordplay.
I cannot really see how this works, but
'saint' could be 'paul' (St Paul) and 'paul' is found within the answer.
'hold' could be 'own' (I've seen this in another clue) and 'own' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'a' is within the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?