Draws the line unwaveringly (5)
I believe the answer is:
rules
'draws' is the definition.
(ruling is a kind of drawing)
'line unwaveringly' is the wordplay.
'line' becomes 'rule' (I've seen this in another clue).
'unwaveringly' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'rule'+'s'='RULES'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for rules that I've seen before include "game in Australia" , "instructions" , "controls" , "strips" , "Regulations, precepts" .)