Oppressed, unhappy staff hiding in back of secret lair (11)
I believe the answer is:
downtrodden
'oppressed' is the definition.
(synonyms)
'unhappy staff hiding in back of secret lair' is the wordplay.
'unhappy' becomes 'down' (I've seen this before).
'staff' becomes 'rod' (both can mean a stick or cane).
'hiding in' is an insertion indicator.
'back of' says to take the final letters.
'lair' becomes 'den' (synonyms).
The last letter of 'secret' is 't'.
't'+'den'='tden'
'rod' going within 'tden' is 'trodden'.
'down'+'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" , "exploited?" , "Subservient" , "Subjugated" .)