Possibly free herbal extract in a cigarette (6)
I believe the answer is:
reefer
'cigarette' is the definition.
(a reefer is a cannabis cigarette)
'possibly free herbal extract' is the wordplay.
'possibly' indicates an anagram.
'herbal extract' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'free' is an anagram of 'reef'.
'reef'+'er'='REEFER'
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for reefer that I've seen before include "A sailor's jacket or marijuana cigarette" , "Unofficial smoking" , "refrigerated transport" , "Easy" , "Canvas trimmer" .)