Crossword Genius

PM once seen at basic military training ground (9,5)

Ross

I believe the answer is:

salisbury plain

I'm a little stuck... Click here to teach me more about this clue!

'military training ground' is the definition.
(I've seen this before)

'pm once seen at basic' is the wordplay.
'pm' becomes 'salisbury' (I've seen this before).
'once seen at basic' becomes 'plain' (I can't explain this - if you can you should give a lot more credence to this answer).
'salisbury'+'plain'='SALISBURY PLAIN'

Can you help me to learn more?

(Other definitions for salisbury plain that I've seen before include "army training area" , "military training ground" , "scene of military manoeuvres" , "Location of Stonehenge" , "military training area" .)

I've seen this clue in The Independent.
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