PM once seen at basic military training ground (9,5)
I believe the answer is:
salisbury plain
'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" .)