Stage in river one in rising mud (6)
I believe the answer is:
podium
'stage' is the definition.
(I've seen this before)
'river one in rising mud' is the wordplay.
'river' becomes 'Po' (Italian river).
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'mud' in reverse letter order is 'dum'.
'i' going inside 'dum' is 'dium'.
'po'+'dium'='PODIUM'
'in' is the link.
(Other definitions for podium that I've seen before include "Platform or pedestal" , "Position of speaker" , "Soapbox" , "Conductor's rostrum" , "Stage" .)