Let in again (7)
I believe the answer is:
readmit
'let in again' is the definition.
('admit' can be a synonym for 'let in' so perhaps 'readmit' is OK for 'let in again')
This is the entire clue.
(Other definitions for readmit that I've seen before include "Welcome back" , "Again accept" , "Dear Tim (anag.)" , "Let back in" , "Let in a second time" .)