Addict returns with essentially filthy product (6)
I believe the answer is:
result
'product' is the definition.
(I've seen this in another clue)
'addict returns with essentially filthy' is the wordplay.
'addict' becomes 'user' (drug user).
'returns' is a reversal indicator.
'with' says to put letters next to each other.
'essentially' says to take the centre.
The middle letters of 'filthy' are 'lt'.
'user' written backwards gives 'resu'.
'resu'+'lt'='RESULT'
(Other definitions for result that I've seen before include "Ulster (anag) - outcome" , "Effect of lustre" , "'Outcome, consequence (6)'" , "Effect - outcome" , "Final score" .)