Twosome united in act (3)
I believe the answer is:
duo
'twosome' is the definition.
(synonyms)
'united in act' is the wordplay.
'united' becomes 'U' (abbreviation).
'in' means one lot of letters goes inside another.
'act' becomes 'do' (to do is to act or execute a task).
'u' put within 'do' is 'DUO'.
(Other definitions for duo that I've seen before include "Performing couple" , "Musical pair" , "Brace" , "rejected Arab's lute" , "Twosome" .)