Crossword Genius

A variable soldier had a royal audience (9)

Ross

I believe the answer is:

parameter

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'a variable' is the definition.
(parameter can mean a variable or value)

'soldier had a royal audience' is the wordplay.
'soldier' becomes 'para' (short for paratrouper).
'had' means one lot of letters go next to another.
'a royal audience' becomes 'meter' (I can't explain this - if you can you should give a lot more credence to this answer).
'para'+'meter'='PARAMETER'

Can you help me to learn more?

(Other definitions for parameter that I've seen before include "One of a set of factors as conditions for operation of a system" , "Limiting factor" , "Limitation" , "Quantifiable characteristic of a system" , "Factor defining limits" .)

I've seen this clue in The Daily Express.
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