Reserve can control movements in a wide area (12)
I believe the answer is:
subcontinent
'wide area' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'reserve can control movements in a' is the wordplay.
I cannot really see how this works, but
'reserve' could be 'sub' (I've seen this in other clues) and 'sub' is present in the answer.
'can' could be 'tin' (both are metal containers) and 'tin' is present in the answer.
'a' could be 'one' and 'one' is present in the remaining letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for subcontinent that I've seen before include "Large land mass" , "Area such as India" .)