Round rings, circle left in region on base of nipple (7)

'round rings' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.

'circle left in region on base of nipple' is the wordplay.
'circle' becomes 'o' ().
'left in' becomes 'l' (I've seen this in other clues).
'region' becomes 'area' (area is a kind of region).
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'base of' suggests the final letters.
The final letter of 'nipple' is 'e'.
'o'+'l'='ol'
'ol' put inside 'area' is 'areola'.
'areola'+'e'='AREOLAE'

Can you help me to learn more?