Former PM with detailed scheme involving one military training area (9,5)
I believe the answer is:
salisbury plain
'military training area' is the definition.
(I've seen this before)
'former pm with detailed scheme involving one' is the wordplay.
'former pm' becomes 'salisbury' (Marquess of Salisbury).
'with' is a charade indicator (letters next to each other).
'detailed scheme' becomes 'plan'.
'involving' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'plan' going around 'i' is 'plain'.
'salisbury'+'plain'='SALISBURY PLAIN'
(Other definitions for salisbury plain that I've seen before include "Location of Stonehenge" , "scene of military manoeuvres" , "army training area" , "part of Wiltshire" , "military training ground" .)