One way of underlining a decision (6)
I believe the answer is:
ruling
I believe this clue is a double definition.
'one way of underlining' is the first definition.
(ruling is underlining with a ruler)
'a decision' is the second definition.
(thesaurus)
(Other definitions for ruling that I've seen before include "judgment" , "Judgement" , "Judge's decision" , "Authoritative decision" , "Court decision" .)