Owl, might one say: once an opponent of the king? (15)
I believe the answer is:
parliamentarian
I believe this is a double definition.
'owl might one say' is the first definition.
(parliament can mean a group of owls)
'once an opponent of the king?' is the second definition.
(opponent of Charles I in the English Civil War)
(Other definitions for parliamentarian that I've seen before include "politician" , "Eg, an MP" , "elected representative?" , "legislator" .)