Key frequently used to get in (5)
This clue could be a double definition.
'key frequently' is the first definition.
'enter' can be an answer for 'key' (key on a computer keyboard). I'm not certain of the 'frequently' bit.
'to get in' is the second definition.
('enter' can be a synonym of 'get in')
'used' is the link.
Can you help me to learn more?
(Other definitions for enter that I've seen before include "Inscribe", "Go/come in", "Break in", "Register (as a competitor)", "Record (in a log)".)