Holding catch over river (6)
I believe the answer is:
tenure
'holding' is the definition.
(thesaurus)
'catch over river' is the wordplay.
'catch' becomes 'net' (netting is a kind of catching).
'over' says the letters should be written in reverse.
'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 "Term of office" , "Length of time a position is held" , "Residence" , "Neuter (anag)" , "Term for which to hold a position" .)