Midshipman Easy’s head used in advert (6)
I believe the answer is:
reefer
'midshipman' is the definition.
(I've seen this before)
'easy's head used in advert' is the wordplay.
'easy's head' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'used in' is an insertion indicator.
'advert' becomes 'refer ' (I've seen this in another clue).
'e' inserted into 'refer' is 'REEFER'.
Can you help me to learn more?
(Other definitions for reefer that I've seen before include "Jack < 's joint" , "packed with cannabis?" , "Type of jacket - smoke" , "roll of hemp" , "Double-breasted jacket for a man" .)