One has to sail back using an assumed name (5)
I believe the answer is:
alias
'an assumed name' is the definition.
('alias' can be a synonym of 'assumed name')
'one has to sail back' is the wordplay.
'one has to' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'back' says the letters should be written in reverse.
'sail' back-to-front is 'lias'.
'a'+'lias'='ALIAS'
'using' is the link.
Can you help me to learn more?
(Other definitions for alias that I've seen before include "Assumed identity" , "aka" , "one's assumed" , "Otherwise - an assumed name" , "Alternative identity" .)