Papers in flat about drug used to support testimony (8)
I believe the answer is:
evidence
'testimony' is the definition.
(testimony is evidence given in court)
'papers in flat about drug' is the wordplay.
'papers' becomes 'ID' (identifying documents are one's 'papers').
'in' is an insertion indicator.
'flat' becomes 'even' (level).
'about drug' becomes 'ce' (I am not sure about this - if you are sure you should believe this answer much more).
'id' inserted inside 'even' is 'eviden'.
'eviden'+'ce'='EVIDENCE'
'used to support' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for evidence that I've seen before include "Testimony in a court of law" , "Witness's testimony" , "Facts presented in court" , "Signs, indications" , "Data" .)