Fast getting smashed! (5)
I believe the answer is:
tight
I believe this is a double definition.
'fast' is the first definition.
(I've seen this before)
'smashed' is the second definition.
('tight' can be a synonym of 'smashed')
'getting' acts as a link.
(Other definitions for tight that I've seen before include "Strictly imposed; drunk" , "Stretched" , "Parsimonious" , "corked?" , "Drunk, firm, concise" .)