Flotillas sail anticlockwise but are breached by aggressor (9)

I believe the answer is:

assailant

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

'aggressor' is the definition.
(synonyms)

'flotillas sail anticlockwise but are breached' is the wordplay.
'ASSAILANT' is hidden within the letters.
But, I'm not sure how this is indicated.

'by' acts as a link.

Can you help me to learn more?

(Other definitions for assailant that I've seen before include "One attacking another" , "Aggressive sort" , "Aggressor" , "Who would tear into" , "who is on the offensive?" .)

I've seen this clue in The Irish Times.

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