Oddball’s party beside dam (6)
I believe the answer is:
weirdo
'oddball's' is the definition.
(I've seen this in another clue)
'party beside dam' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'dam' becomes 'weir' (a weir is a type of dam).
'do' after 'weir' is 'WEIRDO'.
(Other definitions for weirdo that I've seen before include "Strange person (colloq.)" , "Fruitcake" , "Eccentric type" , "strange character" , "Bizarre character" .)