Evaluates drug injected into rodents (5)
I believe the answer is:
rates
'evaluates' is the definition.
(rating is a kind of evaluating)
'drug injected into rodents' is the wordplay.
'drug' becomes 'e' (E is another name for ecstasy).
'injected into' indicates putting letters inside.
'rodents' becomes 'rats' (rat is a kind of rodent).
'e' going into 'rats' is 'RATES'.
(Other definitions for rates that I've seen before include "classes" , "Fees" , "Former means of local taxation" , "Property-based source of local revenue" , "estimates?" .)