Decided where to draw the line? (5)

I believe the answer is:
ruled
'decided' 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 'd' (I can't explain this - if you can you should believe this answer much more).
'rule'+'d'='RULED'
'where' is the link.
Can you help me to learn more?
(Other definitions for ruled that I've seen before include "Governed, used one's authority" , "Reigned or ordered" , "Held the reins" , "(Of writing paper) having printed lines" , "Made authoritative statement" .)
