Ruling to wane, when being ignored (6)
I believe the answer is:
decree
'ruling' is the definition.
(a decree is a ruling or judgment)
'wane when being ignored' is the wordplay.
'wane' becomes 'decrease' (to wane is to decrease or reduce).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'being ignored' indicates named letters should be taken away.
'decrease' with 'as' removed is 'DECREE'.
'to' acts as a link.
(Other definitions for decree that I've seen before include "An official proclamation or ruling" , "Ordain" , "Legally-binding edict" , "Recede following an official order" , "Judgment of a court" .)