Fix of heroin ultimately leading to trouble (4)
I believe the answer is:
nail
'fix' is the definition.
(thesaurus)
'heroin ultimately leading to trouble' is the wordplay.
'ultimately' says to take the final letters.
'leading to' means one lot of letters go next to another (some letters go before or lead others).
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
The final letter of 'heroin' is 'n'.
'n'+'ail'='NAIL'
'of' acts as a link.
(Other definitions for nail that I've seen before include "SECOND PART OF 17" , "one wouldn't want in one's coffin?" , "Apprehend" , "Collar -- cop" , "Brad, say" .)