Not an exclusive thing to do, you'll admit (3,2)
I believe the answer is:
let in
'admit' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for let in that I've seen before include "Admit" , "Grant entry" , "swindle . . ." , "Allowed to enter" , "Allow to enter" .)