Setter’s regrettably served up highly seasoned sausage (6)
I believe the answer is:
salami
'highly seasoned sausage' is the definition.
(I've seen this in another clue)
'setter's regrettably served up' is the wordplay.
'setter's' becomes 'im' (I am - referring to the crossword setter).
'regrettably' becomes 'alas' ('alas' is said before regrettable information).
'served up' says the letters should be written backwards (in a down clue, going upwards).
'im'+'alas'='imalas'
'imalas' back-to-front is 'SALAMI'.
(Other definitions for salami that I've seen before include "Highly seasoned Italian sausage" , "Garlic- flavoured sausage" , "Highly-spiced sausage" , "meat product" , "Highly-seasonal sausage" .)