Useless father takes drug - smack (8)
I believe the answer is:
deadbeat
'useless father takes drug smack' is the definition.
I can't judge whether this defines the answer.
'father takes drug smack' is the wordplay.
'father' becomes 'dad' (informal term for father).
'takes' indicates putting letters inside.
'drug' becomes 'e' (E is another name for ecstasy).
'smack' becomes 'beat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dad' enclosing 'e' is 'dead'.
'dead'+'beat'='DEADBEAT'
Can you help me to learn more?
(Other definitions for deadbeat that I've seen before include "lazy so-and-so" , "Lazy, useless person" , "idle so-and-so" , "extremely tired" , "idler" .)