Cat in agony given drug, a chemical compound (8)
I believe the answer is:
ptomaine
'a chemical compound' is the definition.
(ptomaine is a kind of chemical compound)
'cat in agony given drug' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'in' is an insertion indicator.
'agony' becomes 'pain' (I've seen this in another clue).
'given' is a charade indicator (letters next to each other).
'drug' becomes 'e' (E is another name for ecstasy).
'tom' going within 'pain' is 'ptomain'.
'ptomain'+'e'='PTOMAINE'
(Other definitions for ptomaine that I've seen before include "Old term for food poisoning" , "Oddly, a pimento might cause food poisoning" , "Consequence of decay" , "poison!" , "A pimento might turn to food poisoning" .)