Drug used illegally seen in training is a trace in blood (8,7)
I believe the answer is:
anabolic steroid
'drug used illegally seen in' is the definition.
The definition suggests a singular noun which matches the answer.
'training is a trace in blood' is the wordplay.
'training' indicates anagramming the letters (to train can mean to grow something into a shape).
'is'+'a'+'trace'+'in'+'blood'='isatraceinblood'
'isatraceinblood' is an anagram of 'ANABOLIC STEROID'.
Can you help me to learn more?
(Other definitions for anabolic steroid that I've seen before include "this may provide therapeutic treatment" , "performance enhancer" , "in the event it's banned" .)