Calming drug - new vet's idea (8)
I believe the answer is:
sedative
'calming drug' is the definition.
(I've seen this before)
'new vet's idea' is the wordplay.
'new' indicates anagramming the letters.
'vets'+'idea'='vetsidea'
'vetsidea' is an anagram of 'SEDATIVE'.
(Other definitions for sedative that I've seen before include "Tranquillising drug" , "Deviates from calming medicine" , "Tranquilising drug" , "quieter" , "Drug to make one sleep" .)