Couple act united within (3)
I believe the answer is:
duo
'couple' is the definition.
(two people)
'act united within' is the wordplay.
'act' becomes 'do' (to do is to act or execute a task).
'united' becomes 'U' (as in Man U, English football team).
'within' means one lot of letters goes inside another.
'do' enclosing 'u' is 'DUO'.
(Other definitions for duo that I've seen before include "Couple, pair" , "Pair of performers" , "Pair of musicians" , "Two of a kind" , "Pair (of people, things)" .)