Eccentric having party by river dam (6)
I believe the answer is:
weirdo
'eccentric' is the definition.
(thesaurus)
'party by river dam' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'by' is a charade indicator (letters next to each other).
'river dam' becomes 'weir' (I've seen this before).
'do' after 'weir' is 'WEIRDO'.
'having' is the link.
(Other definitions for weirdo that I've seen before include "Crank - oddball" , "strange character" , "Fruitcake" , "Derogatory term for an eccentric" , "Nutcase" .)