Two complete circles united (3)
I believe the answer is:
duo
'two' is the definition.
(duo is a kind of two)
'complete circles united' is the wordplay.
'complete' becomes 'do' (doing is a kind of completing).
'circles' is an insertion indicator.
'united' becomes 'u'.
'do' placed around 'u' is 'DUO'.
(Other definitions for duo that I've seen before include "entertainers such as Eric and Ernie" , "Item" , "Pair of people/things" , "Pair of singers" , "They might play" .)