Investigate European vote in county (6)
I believe the answer is:
sussex
'county' is the definition.
(I have seen 'Former county' mean 'sussex' so perhaps 'county' could also mean 'sussex')
'investigate european vote' is the wordplay.
'investigate' becomes 'suss' (to suss out something is to discover it).
'european' becomes 'e' (abbreviation e.g. EU).
'vote' becomes 'x' (how votes are marked).
'suss'+'e'+'x'='SUSSEX'
'in' acts as a link.
(Other definitions for sussex that I've seen before include "County (maybe speckled)" , "East and West English county" , "Former county, now divided East and West" , "Historic S. coast county" , "County on the English channel" .)