Being slushy drove one mad (11)
I believe the answer is:
sentimental
'being slushy' is the definition.
('sentimental' can be a synonym of 'slushy')
'drove one mad' is the wordplay.
'drove' becomes 'sent' (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'mad' becomes 'mental'.
'sent'+'i'+'mental'='SENTIMENTAL'
(Other definitions for sentimental that I've seen before include "Expressing tender emotions" , "Mawkish" , "With tender, nostalgic or sad feelings" , "Soft-hearted; romantic" , "Appealing to emotion rather than actuality" .)