Gain admission (5)
'gain admission' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for enter that I've seen before include "Make a record of" , "Get in" , "Introduce" , "Register (as a competitor)" , "Come/go in" .)
(Other definitions for get in that I've seen before include "Effect entrance (3,.2)" , "Enter" , "secure access" .)