A sail used under another name (5)
I believe the answer is:
alias
'another name' is the definition.
(I've seen this in another clue)
'a sail used under' is the wordplay.
'used' indicates an anagram.
'under' says to put letters next to each other (in a down clue, some letters go below others).
'sail' is an anagram of 'lias'.
'a'+'lias'='ALIAS'
(Other definitions for alias that I've seen before include "Alternative identity" , "Otherwise - an assumed name" , "it may be assumed" , "Assumed identity" , "Handle that isn't genuine" .)