Guard missing second way in (5)
I believe the answer is:
entry
'way in' is the definition.
'guard missing second' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'missing' indicates named letters should be taken away.
'second' becomes 's' (common abbreviation).
'sentry' with 's' taken out is 'ENTRY'.
(Other definitions for entry that I've seen before include "Opening" , "candidate" , "Admittance" , "Admission" , "vestibule" .)