Now dance party is under a sun roof, it'll get bigger and bigger (8)
I believe the answer is:
snowball
'get bigger and bigger' is the definition.
(I've seen this in another clue)
'now dance party is under a sun roof' is the wordplay.
'dance party' becomes 'ball' (I can't justify this - if you can you should believe this answer much more).
'is under' means one lot of letters go next to another (below, in a down clue).
'roof' suggests taking the first letters (the letter at the top of the word).
The initial letter of 'sun' is 's'.
'now'+'ball'='nowball'
'nowball' after 's' is 'SNOWBALL'.
'it'll' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for snowball that I've seen before include "It's thrown in winter fun (8)" , "Cocktail made from lemonade and advocaat" , "Drink of advocaat and lemonade" , "Increase rapidly in size - advocaat and lemonade" , "Heroin and cocaine" .)