British naval base at the harbour's entrance (10)
I believe the answer is:
portsmouth
'british naval base' is the definition.
The definition suggests a singular noun which matches the answer.
'at the harbour's entrance' is the wordplay.
'at' is a charade indicator (letters next to each other).
'the harbour's' becomes 'ports'.
'entrance' becomes 'mouth' (eg the mouth of a cave).
'ports' put next to 'mouth' is 'PORTSMOUTH'.
Can you help me to learn more?
(Other definitions for portsmouth that I've seen before include "South coast city" , "naval base" .)