Avoids holding appeal backing drug (8)
I believe the answer is:
sedative
'drug' is the definition.
(sedative is a kind of drug)
'avoids holding appeal backing' is the wordplay.
'avoids' becomes 'evades' (evading is a kind of avoiding**).
'holding' is an insertion indicator.
'appeal' becomes 'it'.
'backing' shows that the letters should be reversed in order.
'evades' enclosing 'it' is 'evitades'.
'evitades' reversed gives 'SEDATIVE'.
(Other definitions for sedative that I've seen before include "Dates I've changed for a tranquiliser" , "Deviates from calming medicine" , "Depressant" , "Tranquilliser" , "Drug to make one sleep" .)