A variable soldier had a royal audience (9)
I believe the answer is:
parameter
'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" .)