Improvised epilogue about taking heroin (2,3)
I believe the answer is:
ad hoc
'improvised' is the definition.
(I've seen this in another clue)
'epilogue about taking heroin' is the wordplay.
'epilogue' becomes 'coda' (I've seen this in another clue).
'about' is a reversal indicator.
'taking' indicates putting letters inside.
'heroin' becomes 'H' (synonyms).
'coda' written backwards gives 'adoc'.
'adoc' going around 'h' is 'AD HOC'.
(Other definitions for ad hoc that I've seen before include "Done for a particular purpose only" , "Applied to a particular situation" , "particular case" , "'Improvised or impromptu (2,3)'" , "specific occasion" .)