As nice as pie about Romeo (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'nice as pie about romeo' is the wordplay.
'pie' is an anagram indicator.
'about' indicates putting letters inside.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'nice'+'as'='niceas'
'niceas' is an anagram of 'asenic'.
'asenic' enclosing 'r' is 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "killer" , "Brittle grey-green semi-metal, atomic number 33" , "Powerful poison" , ", it goes with old lace" , "Poisons in scare" .)