It makes a straight line for the king (5)
I believe the answer is:
ruler
This might be a double definition.
'it makes a straight line' is the first definition.
The definition suggests a singular noun which matches the answer.
'the king' is the second definition.
(thesaurus)
'for' acts as a link.
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Aid for a straight line" , "Measuring rod; governor" , "Aid to drawing straight lines" , "Source of line" , "Straight edge; governor" .)