Guard removes top to gain access (5)
I believe the answer is:
entry
'access' is the definition.
(I know that entry is a type of access)
'guard removes top' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'removes top' means to remove the first letter.
'sentry' with its first letter taken off is 'ENTRY'.
'to gain' is the link.
(Other definitions for entry that I've seen before include "List of candidates" , "vestibule" , "Access point" , "Competitor" , "Admittance" .)