Drab regime ultimately established by doctor (3,4)
I believe the answer is:
bed rest
'doctor' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'drab regime ultimately established' is the wordplay.
'ultimately' says to take the final letters.
'established' becomes 'drest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The last letters of 'drab regime' are 'be'.
'be'+'drest'='BED-REST'
'by' is the link.
Can you help me to learn more?
(Another definition for bed rest that I've seen is " sort of treatment".)