Man on river for jolly old party (8)
I believe the answer is:
falstaff
'jolly old party' is the definition.
(jolly character in several Shakespeare plays)
'man on river for' is the wordplay.
'man' becomes 'staff' (both can mean to work at a particular post).
'on' is a charade indicator (letters next to each other).
'river for' becomes 'fal' (I am not sure about this - if you are sure you should believe this answer much more).
'staff' after 'fal' is 'FALSTAFF'.
(Other definitions for falstaff that I've seen before include "Shakespeare's Sir John" , "stout fellow" , "Shakespeare's fat alcoholic knight" , "from 6 &12 [VERDI & SHAKESPEARE] ?" , "Sir John - (MWOW)" .)