Oddball has a party under a dam (6)
I believe the answer is:
weirdo
'oddball' is the definition.
(I've seen this before)
'a party under a dam' is the wordplay.
'a party' becomes 'do' (do is a kind of party).
'under' means one lot of letters go next to another (below, in a down clue).
'a dam' becomes 'weir' (a weir is a type of dam).
'do' after 'weir' is 'WEIRDO'.
'has' is the link.
(Other definitions for weirdo that I've seen before include "Bizarre character" , "Slang for odd or eccentric person" , "Strange person (colloq.)" , "Nutcase" , "Fruitcake" .)