Jennifer finally leaving as Johnson was dressing (4)
I believe the answer is:
mayo
'dressing' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
(Other definitions for mayo that I've seen before include "Thick creamy dressing" , "You'll find it in the Emerald Isle" , "Cold, creamy sauce (abbr)" , "Irish county -- salad cream" , "County of north-west Ireland" .)