Barge in season, heading off south east, beyond river (9)
I believe the answer is:
interpose
'barge in' is the definition.
(thesaurus)
'season heading off south east beyond river' is the wordplay.
'season' becomes 'winter' (winter is a season).
'heading off' means to remove the first letter.
'south east' becomes 'SE' (abbreviation).
'beyond' means one lot of letters go next to another.
'river' becomes 'Po' (Italian river).
'winter' with its first letter removed is 'inter'.
'se' put after 'po' is 'pose'.
'inter'+'pose'='INTERPOSE'
(Other definitions for interpose that I've seen before include "step in" , "Muscle in" , "butt in" , "Place between" , "mediate" .)