As nice as Ritz's top cocktail (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'nice as ritz's top cocktail' is the wordplay.
'top' indicates taking the first letters.
'cocktail' indicates an anagram.
The first letter of 'ritzs' is 'r'.
'nice'+'as'+'r'='niceasr'
'niceasr' is an anagram of 'ARSENIC'.
(Other definitions for arsenic that I've seen before include ", it goes with old lace" , "Poison used in pesticides and weedkillers" , "Element with many poisonous compounds" , "Poison found in races" , "Toxic metallic element used as rat poison" .)