Thought about tailless fish from English port (6,4)

I believe the answer is:

mulled over

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'thought' is the definition.
(mulling over is a kind of thinking)

'tailless fish from english port' is the wordplay.
'tailless fish' becomes 'mull' (I am not sure about this - if you are sure you should believe this answer much more).
'from' becomes 'e' (I've seen this in other clues).
'english port' becomes 'dover' (I've seen this before).
'mull'+'e'+'dover'='MULLED OVER'

'about' acts as a link.

Can you help me to learn more?

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

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