Nags founding member of Stones to get on heroin (6)
I believe the answer is:
horses
'nags' is the definition.
'get on heroin' is the wordplay.
'get' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'heroin' becomes 'horse' (slang term for heroin).
's' put after 'horse' is 'HORSES'.
'founding member of stones to' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for horses that I've seen before include "Equine beasts" , "Hoofed mammals (which go with 19 4)" , "Steeds" , "''If wishes were . . . . . ., beggars would ride''" , "Equine animals" .)