Admission from guard, not the first (5)
I believe the answer is:
entry
'admission' is the definition.
(thesaurus)
'guard not the first' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'not the first' means to remove the first letter.
'sentry' with its initial letter taken off is 'ENTRY'.
'from' is the link.
(Other definitions for entry that I've seen before include "Item inserted in a written record" , "all runners" , "Item in an account book" , "List of candidates" , "Admittance" .)