Overseer exceptionally determined to retain private room (14)
I believe the answer is:
superintendent
'overseer' is the definition.
(synonyms)
'exceptionally determined to retain private room' is the wordplay.
'exceptionally' becomes 'super' (eg super bright means exceptionally bright).
'determined' becomes 'intent' (I've seen this before).
'to retain' indicates putting letters inside.
'private room' becomes 'den' (a secret room or location).
'intent' going around 'den' is 'intendent'.
'super'+'intendent'='SUPERINTENDENT'
(Other definitions for superintendent that I've seen before include "Police chief" , "One who is in charge of an organisation" , "Overseer; police officer" , "Police rank" , "Senior police officer" .)