A sail used under another name (5)
I believe the answer is:
alias
'another name' is the definition.
(I've seen this before)
'a sail used under' is the wordplay.
'used' is an anagram indicator.
'under' is a charade indicator (letters next to each other).
'sail' with letters rearranged gives '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" .)