Displays some Essex postcodes (5)
I believe the answer is:
expos
'displays' is the definition.
(I've seen this before)
'some essex postcodes' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'EXPOS' is hidden within 'essex postcodes'.
(Other definitions for expos that I've seen before include "Public displays" , "International exhibitions" , "International shows" .)