Farm animal I'd put on drug, a chemical compound (5)
I believe the answer is:
oxide
'a chemical compound' is the definition.
(oxide is a kind of chemical compound)
'farm animal i'd put on drug' is the wordplay.
'farm animal' becomes 'ox'.
'put on' says to put letters next to each other.
'drug' becomes 'e' (e is another name for ecstasy).
'ox'+'id'+'e'='OXIDE'
(Other definitions for oxide that I've seen before include "Compound of oxygen and one other element" , "Eg, rust" , "Oxygenous compound" , "Compound of two elements, one with atomic number 8" , "Compound such as rust" .)