Chemical upsets hen a lot (7)
I believe the answer is:
ethanol
'chemical' is the definition.
(thesaurus)
'upsets hen a lot' is the wordplay.
'upsets' indicates an anagram.
'hen'+'a'+'lot'='henalot'
'henalot' with letters rearranged gives 'ETHANOL'.
(Other definitions for ethanol that I've seen before include "constituent for a dram?" , "Kind of alcohol - not hale (anag)" , "volatile flammable substance" , "Intoxicant in wine, spirits, etc" , "Form of alcohol" .)