Key often used to get in (5)
I believe the answer is:
enter
I believe this is a double definition.
'key often used' is the first definition.
(computer keyboard key)
'get in' is the second definition.
(synonyms)
'to' is the link.
(Other definitions for enter that I've seen before include "A key on the right" , "Enlist, join up" , "Make a record of" , "Record - stage direction - come in" , "Fruit drink" .)