Ballasts top galleys, port and starboard (5)

I believe the answer is:

brags

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'port and starboard' is the definition.
I can't judge whether this definition defines the answer.

'ballasts top galleys' is the wordplay.
'top' says to take the initial letters.
'galleys' becomes 'rags' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'ballasts' is 'b'.
'b'+'rags'='BRAGS'

Can you help me to learn more?

(Other definitions for brags that I've seen before include "Boasts" , "Crows" .)

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

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