As one worried Essex man must cast vote (2,5)
I believe the answer is:
'as one' is the definition.
(both can mean as a group)
'worried essex man must cast vote' is the wordplay.
'worried' indicates anagramming the letters.
'must cast' indicates named letters should be taken away.
'vote' becomes 'x' (how votes are marked).
'essexman' with 'x' taken away is 'esseman'.
'esseman' with letters rearranged gives 'EN MASSE'.
(Other definitions for en masse that I've seen before include "As a body (Fr.)" , "as a bunch" , "All together - in France (2.5)" , "All together (Fr.)" , "In force" .)