Place for charter party not set: Imogen finally picked venue (9)
I believe the answer is:
runnymede
'place for charter party' is the definition.
(site of the signing of the Magna Carta)
'not set imogen finally picked venue' is the wordplay.
'not set' becomes 'runny' (I've seen this before).
'imogen' becomes 'me' (I can't justify this - if you can you should give a lot more credence to this answer).
'finally' suggests the final letters.
The last letters of 'picked venue' are 'de'.
'runny'+'me'+'de'='RUNNYMEDE'
(Other definitions for runnymede that I've seen before include "Place in Surrey" , "Democratic convention site" , "Here the English got charter" , "Magna Carta meadow" , "riverside location" .)