Oddball having party by low dam (6)
I believe the answer is:
weirdo
'oddball' is the definition.
(I've seen this before)
'party by low dam' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'by' is a charade indicator (letters next to each other).
'low dam' becomes 'weir' (I've seen this in another clue).
'do' put after 'weir' is 'WEIRDO'.
'having' acts as a link.
(Other definitions for weirdo that I've seen before include "Informally eccentric person - sounds spooky" , "Unpleasantly strange or eccentric person" , "Crank - oddball" , "Bizarre character" , "An eccentric (sl.)" .)