Tonic in US mixed with last of finest malt (9)
I believe the answer is:
stimulant
'tonic' is the definition.
(I've seen this before)
'in us mixed with last of finest malt' is the wordplay.
'mixed with' indicates an anagram.
'last of' indicates one should take the final letters.
The final letter of 'finest' is 't'.
'in'+'us'='inus'
't'+'malt'='tmalt'
'inus' anagrammed with 'tmalt' can make 'STIMULANT'.
(Other definitions for stimulant that I've seen before include "A drug or drink to make one more active" , "Booster" , "Substance to increase activity" , "Something promoting activity" , "Means of prompting action" .)