Poisonous element nicer, as improved (7)
I believe the answer is:
arsenic
'poisonous element' is the definition.
(I've seen this before)
'nicer as improved' is the wordplay.
'improved' is an anagram indicator.
'nicer'+'as'='niceras'
'niceras' is an anagram of 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "killer" , "Element No 33" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Element with many poisonous compounds" , "Can rise result from poison?" .)