Calming drug is vet's idea possibly (8)
I believe the answer is:
sedative
'calming drug' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sedative that I've seen before include "Dates I've changed for a tranquiliser" , "quieter" , "Depressant" , "Tranquilising drug" , "having soothing effect" .)