Substance used in test run on key worker (7)
I believe the answer is:
reagent
'substance used in test' is the definition.
I can't judge whether this definition defines the answer.
'run on key worker' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'on key' becomes 'e' (I've seen this in other clues. I am not sure about the 'on' bit.).
'worker' becomes 'agent' (agent is a kind of worker).
'r'+'e'+'agent'='REAGENT'
Can you help me to learn more?
(Other definitions for reagent that I've seen before include "What causes change" , "Chemically-active substance" , "Get near (anag) - substance used in chemistry" , "Chemical substance used in tests" , "Substance with characteristic reactions, used as a chemical test" .)