Drug user finally caught in Tube (5)
I believe the answer is:
horse
'drug' is the definition.
(horse is a slang term for heroin)
'user finally caught in tube' is the wordplay.
'user finally' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'caught in' means one lot of letters goes inside another.
'tube' becomes 'hose' (hose is a kind of tube).
'r' inserted into 'hose' is 'HORSE'.
Can you help me to learn more?
(Other definitions for horse that I've seen before include "Vaulting block" , "punch, perhaps" , "Animal providing" , "Animal with withers" , "Animal; cavalry" .)