One who submits to control to a limited extent (6)
I believe the answer is:
medium
This clue could be a double definition.
'one who submits to control' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'limited extent' is the second definition.
The answer and definition can be both states of affairs as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'to a' acts as a link.
Can you help me to learn more?
(Other definitions for medium that I've seen before include "Normal sized - agency" , "Person who conducts a siance" , "Average - means of communication" , "Spiritualist who conducts seance" , "TV, say" .)