Work out the other royal title (6)

I believe the answer is:

sussex

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'royal title' is the definition.
The definition suggests a singular noun which matches the answer.

'work out the other' is the wordplay.
'work out' becomes 'sus' (I can't justify this - if you can you should give a lot more credence to this answer).
'the other' becomes 'sex' (old-fashioned slang term).
'sus'+'sex'='SUSSEX'

Can you help me to learn more?

(Other definitions for sussex that I've seen before include "Old county of S. England" , "Tough; like tendons" , "Seaside county" , "English breed of chicken" , "fowl" .)

I've seen this clue in The Guardian.

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