Goes off to catch river monsters (5)
I believe the answer is:
ogres
'monsters' is the definition.
(ogre is a type of monster)
'goes off to catch river' is the wordplay.
'off' indicates anagramming the letters.
'to catch' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'goes' anagrammed gives 'oges'.
'oges' going around 'r' is 'OGRES'.
(Other definitions for ogres that I've seen before include "Monsters can be goers" , "Giants in fairytales" , "Formidable people" , "Bugbears" , "huge figures" .)