It goes round and round, up and down (7)
I believe the answer is:
rotator
'down' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?
'it goes round and round up' is the wordplay.
'it goes round' becomes 'rotor' (I've seen this before).
'and round' means one lot of letters goes inside another (some letters go round others).
'up' becomes 'at' (I am not sure about this - if you are sure you should believe this answer much more).
'rotor' enclosing 'at' is 'ROTATOR'.
'and' is the link.
Can you help me to learn more?
(Other definitions for rotator that I've seen before include "Something that turns" , "Device that spins" , "Something turning" , "One goes around" , "Revolver" .)