Mercury's first in order of deities listed (8)
I believe the answer is:
itemised
'listed' is the definition.
(I've seen this before)
'mercury's first in order of deities' is the wordplay.
'first' says to take the initial letters.
'in' is an insertion indicator.
'order of' indicates an anagram.
The first letter of 'mercurys' is 'm'.
'deities' with letters rearranged gives 'iteised'.
'm' inserted within 'iteised' is 'ITEMISED'.
(Other definitions for itemised that I've seen before include "Ticked off one by one" , "placed on list" , "put on list?" , "Enumerated, listed" , "Given separately in a list" .)