Weirdo backing party finally organised dance (7)
I believe the answer is:
oddball
'weirdo' is the definition.
(I've seen this in another clue)
'backing party finally organised dance' is the wordplay.
'backing' is a reversal indicator.
'party' becomes 'do' (do is a kind of party).
'finally' indicates one should take the final letters.
'dance' becomes 'ball' (ball is a kind of dance).
The final letter of 'organised' is 'd'.
'do' in reverse letter order is 'od'.
'od'+'d'+'ball'='ODDBALL'
(Other definitions for oddball that I've seen before include "Unusual" , "Eccentric person like peculiar sphere" , "Bold lad (anag)" , "Crackpot" , "eccentric type" .)