Powerful warning king must leave (6)
I believe the answer is:
potent
'powerful' is the definition.
(I know that powerful can be written as potent)
'warning king must leave' is the wordplay.
'warning' becomes 'portent' (a portent is a sign or warning).
'king' becomes 'R' (abbreviation of Latin rex).
'must leave' indicates named letters should be taken away.
'portent' with 'r' removed is 'POTENT'.
(Other definitions for potent that I've seen before include "Top Ten (anag.)" , "Forceful" , "Powerful or effective" , "Maybe overpowering" , "Powerful, influential" .)