Like leaving beermat that’s forming circle (7)
I believe the answer is:
coterie
'circle' is the definition.
(I know that coterie is a type of circle)
'like leaving beermat that's' is the wordplay.
I cannot really understand how this works, but
'that's' could be 'ie' and 'ie' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'forming' is the link.
Can you help me to learn more?
(Other definitions for coterie that I've seen before include "EEC trio in exclusive group" , "Small exclusive group of people" , "Small exclusive circle with a common purpose" , "Set" , "Group, band" .)