Some currency back in circulation (4)
I believe the answer is:
eddy
This clue might be a double definition.
'some currency back' is the first definition.
'eddy' can be an answer for 'back' (thesaurus). I am not certain of the 'some currency' bit.
'circulation' is the second definition.
The definition and answer can be both related to natural events as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'in' is the link.
Can you help me to learn more?
(Other definitions for eddy that I've seen before include "Spinning current" , "Swirl, whirl" , "current running back" , "Spin" , "Rapid circular current" .)