One creature of habit! (3)
I believe the answer is:
nun
'creature of habit' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'one' is the wordplay.
I cannot really see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found within the answer.
A single letter 'n' remains 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 nun that I've seen before include "A palindromic lady in religious order" , "Mother Superior, perhaps" , "Devout woman" , "Mother Teresa, maybe" , "Woman with a habit" .)