Sleeping duke matron treated (7)
I believe the answer is:
dormant
'sleeping' is the definition.
(synonyms)
'duke matron treated' is the wordplay.
'duke' becomes 'd'.
'treated' indicates an anagram.
'matron' with letters rearranged gives 'ormant'.
'd'+'ormant'='DORMANT'
(Other definitions for dormant that I've seen before include "Latent" , "Suspended" , "asleep" , "Temporarily quiet" , "Inactive, but not dead" .)