Admission of guard losing seconds (5)
I believe the answer is:
entry
'admission' is the definition.
(I've seen this before)
'guard losing seconds' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'losing' suggests deleting specific letters.
'seconds' becomes 's' (abbreviation).
'sentry' with 's' removed is 'ENTRY'.
'of' is the link.
(Other definitions for entry that I've seen before include "contestant" , "Item in an account book" , "The act of going in" , "Doorway" , "Ingress, portal" .)