Former PM with scheme involving one part of Wiltshire (9,5)
I believe the answer is:
salisbury plain
'one part of wiltshire' is the definition.
(plain in Wiltshire)
'former pm with scheme' is the wordplay.
'former pm' becomes 'salisbury' (Marquess of Salisbury).
'with' says to put letters next to each other.
'scheme' becomes 'plain' (I can't justify this - if you can you should believe this answer much more).
'salisbury'+'plain'='SALISBURY PLAIN'
'involving' is the link.
Can you help me to learn more?
(Other definitions for salisbury plain that I've seen before include "part of Wiltshire" , "scene of military manoeuvres" , "military training ground" , "army training area" , "Location of Stonehenge" .)