Margin of a river, generally (5)
I believe the answer is:
verge
'margin' is the definition.
(verge is a kind of margin)
'of a river generally' is the wordplay.
'of' indicates a hidden word.
'VERGE' can be found hidden inside 'a river generally'.
(Other definitions for verge that I've seen before include "Grassy road edge" , "Margin, edge" , "Grass edge to road" , "emblem of office for bishop" , "Margin or brink" .)