Assesses lab subjects, having injected drug (5)
I believe the answer is:
rates
'assesses' is the definition.
(rating is a kind of assessing)
'lab subjects having injected drug' is the wordplay.
'lab subjects' becomes 'rats' (I've seen this in another clue).
'having injected' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'rats' going around 'e' is 'RATES'.
(Other definitions for rates that I've seen before include "(Old) property taxes" , "Local taxes on commercial property" , "Former means of local taxation" , "Taxes paid by businesses to local authority" , "Speeds, evaluates" .)