Port is a capital place for some frigates (4)
I believe the answer is:
riga
'port' is the definition.
(I know that Riga is a port)
'a capital place for some frigates' is the wordplay.
'RIGA' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
'is' is the link.
Can you help me to learn more?
(Other definitions for riga that I've seen before include "trading centre in Latvia" , "capital in Europe" , "Latvian port" , "European capital" , "A rig in Latvia" .)