Otherwise called a sail? (5)

I believe the answer is:

alias

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'otherwise called' is the definition.
(I've seen this before)

'a sail?' is the wordplay.
'a sail' anagrams to 'ALIAS'.
Though, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for alias that I've seen before include "Otherwise known as" , "Pseudonym" , "Cabbage salad" , "it may be assumed" , "Assumed identity" .)

I've seen this clue in the Evening Standard.

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