Oppressed duke to have pistol hidden in ultimately secret lair (11)
I believe the answer is:
downtrodden
'oppressed' is the definition.
(synonyms)
'duke to have pistol hidden in ultimately secret lair' is the wordplay.
'duke' becomes 'd' (abbreviation).
'to have' becomes 'own' (to have something is to own it).
'pistol' becomes 'rod' (rod is a kind of pistol).
'hidden in' means one lot of letters goes inside another.
'ultimately secret lair' becomes 'tden' (I can't justify this - if you can you should believe this answer much more).
'rod' inserted within 'tden' is 'trodden'.
'd'+'own'+'trodden'='DOWNTRODDEN'
(Other definitions for downtrodden that I've seen before include "Oppressed by people in power" , "Oppressed or treated badly by someone in power" , "Subservient" , "exploited?" , "Subjugated" .)