Monsters finally go looking for three knights (5)
I believe the answer is:
ogres
'monsters' is the definition.
('ogre' can be a synonym of 'monster')
'finally go looking for three knights' is the wordplay.
'finally' indicates one should take the final letters.
The last letters of 'go looking for three knights' are 'OGRES'.
(Other definitions for ogres that I've seen before include "Man-eating giants of folklore" , "She-giant of fairy tale" , "Giant types" , "Awful brutes" , "Bugbears" .)