Decides where to draw the line? (5)
I believe the answer is:
rules
'decides' is the definition.
(ruling is a kind of deciding)
'to draw the line?' is the wordplay.
'to draw' becomes 'rule' (ruling is a kind of drawing).
'the line?' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rule'+'s'='RULES'
'where' is the link.
Can you help me to learn more?
(Other definitions for rules that I've seen before include "See 7" , "is supreme, OK?" , "Governs; regulations" , "Laws; draws lines" , "controls" .)