Eccentric dam party (6)
I believe the answer is:
weirdo
'eccentric' is the definition.
(I've seen this before)
'dam party' is the wordplay.
'dam' becomes 'weir' (a weir is a type of dam).
'party' becomes 'do' (do can informally mean a party).
'weir'+'do'='WEIRDO'
(Other definitions for weirdo that I've seen before include "Fruitcake" , "Strange person (colloq.)" , "Derogatory term for an eccentric" , "Unpleasantly strange or eccentric person" , "An eccentric (sl.)" .)