Naturally he's the one to draw the line (5)
I believe the answer is:
ruler
'naturally he's the one' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'draw the line' is the wordplay.
'draw' becomes 'rule' (ruling is a kind of drawing).
'the line' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'rule'+'r'='RULER'
'to' is the link.
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Person commanding" , "Drawing stick or monarch" , "King, president etc." , "Aid for a straight line" , "Commander" .)