Attack when ahead of fleet (6)

I believe the answer is:

assail

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'attack' is the definition.
(synonyms)

'when ahead of fleet' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'ahead of fleet' becomes 'sail' (I can't explain this - if you can you should give a lot more credence to this answer).
'as'+'sail'='ASSAIL'

Can you help me to learn more?

(Other definitions for assail that I've seen before include "Beset" , "Launch an attack" , "Aggressively attack" , "Charge" , "Trouble" .)

I've seen this clue in The Independent.

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