Worthless Russian fighter turned to hard drug (8)
I believe the answer is:
gimcrack
'worthless' is the definition.
(I've seen this before)
'russian fighter turned to hard drug' is the wordplay.
'russian fighter' becomes 'MiG' (Russian fighter plane manufacturer).
'turned' says the letters should be written backwards.
'to hard drug' becomes 'crack' (crack is a kind of hard drug).
'mig' backwards is 'gim'.
'gim'+'crack'='GIMCRACK'
(Other definitions for gimcrack that I've seen before include "Shoddy; a poorly made article" , "Flashy" , "Tinpot" , "Rubbish!" , "Showy but cheap or badly made" .)