An éclair's fancy - and dangerous to eat (9)
I believe the answer is:
Here is my best explanation:
'dangerous to eat' is the definition.
'an eclair's fancy' is the wordplay.
'fancy' indicates anagramming the letters.
'aneclairs' is an anagram of 'ARSENICAL'.
'and' is the link.
(Other definitions for arsenical that I've seen before include "deadly combination", "Like a certain element", "Mortal" and "containing poison".)