Killer mixed up a small portion (3,6)
I believe the answer is:
rat poison
'killer' is the definition.
(I've seen this before)
'mixed up a small portion' is the wordplay.
'mixed up' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'+'portion'='asportion'
'asportion' anagrammed gives 'RAT-POISON'.
(Other definitions for rat poison that I've seen before include "Assistance in exterminating" , "Rodenticide" , "Vermin control" , "Vermin exterminator" , "eliminator of pests" .)