Truly is an awful cocktail (5,4)
I believe the answer is:
rusty nail
'cocktail' is the definition.
(I've seen this before)
'truly is an awful' is the wordplay.
'awful' indicates an anagram.
'truly'+'is'+'an'='trulyisan'
'trulyisan' is an anagram of 'RUSTY NAIL'.
(Other definitions for rusty nail that I've seen before include "one drunk at cocktail party?" , "one's likely to get hammered!" , "Strong sweet cocktail" .)