Cocaine and heroin? Me? I'm a mythical creature (5,5)
I believe the answer is:
tooth fairy
'a mythical creature' is the definition.
'cocaine and heroin? me?' is the wordplay.
'cocaine' becomes 'toot' (informal term for a snort of cocaine).
'and' means one lot of letters go next to another.
'heroin?' becomes 'h' ('H' can be a synonym of 'heroin').
'me?' becomes 'fairy' (I can't justify this - if you can you should give a lot more credence to this answer).
'toot'+'h'+'fairy'='TOOTH FAIRY'
'i'm' is the link.
This may not be right. It may be part of another bit of the clue.
(Other definitions for tooth fairy that I've seen before include "Parent really" , "It may bring comfort to a child" , "Visitor leaving kids money" , "She leaves money for molar" , "Spirit said to leave money at night in compensation for losing a gnasher" .)