Ruining lawns, lob white missile (8)
I believe the answer is:
snowball
'white missile' is the definition.
(missile can mean anything thrown as a weapon)
'ruining lawns lob' is the wordplay.
'ruining' is an anagram indicator.
'lawns'+'lob'='lawnslob'
'lawnslob' is an anagram of 'SNOWBALL'.
(Other definitions for snowball that I've seen before include "Mushroom" , "A mixture of drugs" , "Increase in importance" , "Increase rapidly in size - advocaat and lemonade" , "Cocktail made from lemonade and advocaat" .)