Porter to fix security (6,4)

I believe the answer is:

bearer bond

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

'security' is the definition.
(I know that bearer bond is a type of bond)

'porter to fix' is the wordplay.
'porter' becomes 'bearer' (I've seen this in another clue).
'to fix' becomes 'bond' (I can't justify this - if you can you should give a lot more credence to this answer).
'bearer'+'bond'='BEARER BOND'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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