Principal port of Western Australia (9)

I believe the answer is:

fremantle

I'm a little stuck... Click here to teach me more about this clue!

'principal port' is the definition.
'fremantle' can be an answer for 'port' (I've seen this before). I am unsure of the 'principal' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for fremantle that I've seen before include "City serving Perth, Australia as its port" , "Australian port" , "Port near Perth (Australia)" , "port appears" .)

I've seen this clue in The Times.

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