Sold a line to the ultimate buyer in private (7)

I believe the answer is:

soldier

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'private' is the definition.
(I know that private is a type of private)

'sold a line to the ultimate buyer' is the wordplay.
'a line' becomes 'ie' (I can't explain this - if you can you should believe this answer much more).
'to the ultimate' suggests the final letters.
The final letter of 'buyer' is 'r'.
'sold'+'ie'+'r'='SOLDIER'

'in' is the link.

Can you help me to learn more?

(Other definitions for soldier that I've seen before include "regular?" , "Member of army" , "Member of the armed forces" , "Military person" , "Person serving in army" .)

I've seen this clue in the Evening Standard.

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