Slip out following a cigarette (6)
I believe the answer is:
Here is my best explanation:
'a cigarette' is the definition.
'slip out following' is the subsidiary indication.
'out' indicates an anagram (out can mean wrong or inaccurate).
'following' becomes 'ff' (abbreviation used in citations).
'slip' is an anagram of 'spli'.
(Other definitions for spliff that I've seen before include "Cannabis joint", "Marijuana fag", "Blow in this tube", "Marijuana cigarette", "Cannabis cigarette", "drug dose", "Reefer (colloq.)", "Pot cigarette (colloq.)", "Dodgy smoke" and "Joint (for smoking)".)