As nice as dancing with Romeo (7)
I believe the answer is:
arsenic
'as' is the definition.
('arsenic' can be a synonym of 'as')
'nice as dancing with romeo' is the wordplay.
'dancing with' is an anagram indicator.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'nice'+'as'='niceas'
'niceas' with its letters mixed up with 'r' makes 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Powerful poison" , "killer" , "Ratsbane" , "Poisons in scare" , ", it goes with old lace" .)