Barge in open site travelling round river (9)
I believe the answer is:
interpose
'barge in' is the definition.
(thesaurus)
'open site travelling round river' is the wordplay.
'travelling' indicates anagramming the letters.
'round' means one lot of letters goes inside another.
'river' becomes 'r'.
'open'+'site'='opensite'
'opensite' with letters rearranged gives 'intepose'.
'intepose' enclosing 'r' is 'INTERPOSE'.
(Other definitions for interpose that I've seen before include "step in" , "mediate" , "Muscle in" , "Place between" , "Insert between (one thing and another)" .)