Lead boxes contaminate oxygen, substance used in clinical trials (7)
I believe the answer is:
placebo
'substance used in clinical trials' is the definition.
I can't judge whether this defines the answer.
'lead boxes contaminate oxygen' is the wordplay.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'boxes' means one lot of letters goes inside another (the outside letters must 'box' inside letters in).
'contaminate' becomes 'lace' (I can't explain this - if you can you should believe this answer much more).
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'pb' going around 'lace' is 'placeb'.
'placeb'+'o'='PLACEBO'
Can you help me to learn more?
(Other definitions for placebo that I've seen before include "Sham medicine used in test or for effect" , "Inactive control used in drug testing" , "Harmless pill" , "Innocuous medication; palliative" , "Measure purely to calm or humour another" .)