Reliable slogger can be functional on heroin (9)
I believe the answer is:
workhorse
'reliable slogger' is the definition.
'workhorse' can be an answer for 'slogger' (thesaurus). I'm not certain of the 'reliable' bit.
'functional on heroin' is the wordplay.
'functional' becomes 'work' (I can't justify this - if you can you should believe this answer much more).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'heroin' becomes 'horse' (slang term for heroin).
'work'+'horse'='WORKHORSE'
'can be' acts as a link.
Can you help me to learn more?
(Other definitions for workhorse that I've seen before include "Punch maybe" , "tireless toiler" , "Reliable toiler" , "giving regular service" , "Person (or machine) dependably slogging away" .)