Wily monarch now imprisoned (7)
I believe the answer is:
knowing
'wily' is the definition.
(thesaurus)
'monarch now imprisoned' is the wordplay.
'monarch' becomes 'king' (king is a kind of monarch).
'imprisoned' means one lot of letters goes inside another.
'king' enclosing 'now' is 'KNOWING'.
(Other definitions for knowing that I've seen before include "Understanding" , "State of awareness" , "Wise" , "Cunning" , "Arch" .)