Figure out former partner is somewhere in south England (6)

I believe the answer is:

sussex

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'somewhere in south england' is the definition.
The definition suggests a singular noun which matches the answer.

'figure out former partner' is the wordplay.
'figure out' becomes 'suss' (I am not sure about this - if you are sure you should believe this answer much more).
'former partner' becomes 'ex' (an ex-partner).
'suss'+'ex'='SUSSEX'

'is' acts as a link.

Can you help me to learn more?

(Other definitions for sussex that I've seen before include "East and West English county" , "Coastal county" , "Old county of S. England" , "Saxon southern county" , "South coast county" .)

I've seen this clue in The Independent.

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