Tip off guard to obtain access (5)
I believe the answer is:
entry
'access' is the definition.
(entry is a kind of access)
'tip off guard' is the wordplay.
'tip off' means to remove the first letter (the first letter or 'tip' is removed).
'guard' becomes 'sentry' (sentry is a kind of guard).
'sentry' with its initial letter taken away is 'ENTRY'.
'to obtain' is the link.
(Other definitions for entry that I've seen before include "vestibule" , "contestant" , "candidate" , "Ingress, portal" , "all runners" .)