Tranquilliser is vet's idea getting doctored (8)
I believe the answer is:
sedative
'tranquilliser' is the definition.
(I've seen this before)
'vet's idea getting doctored' is the wordplay.
'getting' means one lot of letters go next to another.
'doctored' is an anagram indicator.
'vets'+'idea'='vetsidea'
'vetsidea' anagrammed gives 'SEDATIVE'.
'is' is the link.
(Other definitions for sedative that I've seen before include "Calming drug" , "Tranquilising drug" , "Sleeping drug" , "having soothing effect" , "Dates I've changed for a tranquiliser" .)