Holding catch up by river (6)
I believe the answer is:
tenure
'holding' is the definition.
(thesaurus)
'catch up by river' is the wordplay.
'catch' becomes 'net' (netting is a kind of catching).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'by' is a charade indicator (letters next to each other).
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure'**).
'net' backwards is 'ten'.
'ten'+'ure'='TENURE'
(Other definitions for tenure that I've seen before include "(Right of) occupancy" , "Term of office" , "Term for which to hold a position" , "Length of time in office" , "The holding of land or property" .)