Cruise turned up under a false name (5)
I believe the answer is:
alias
'false name' is the definition.
('alias' can be a synonym of 'false name')
'cruise turned up under a' is the wordplay.
'cruise' becomes 'sail' ('sail' can be a synonym of 'cruise').
'turned up' shows that the letters should be reversed in order.
'under' means one lot of letters go next to another.
'sail' in reverse letter order is 'lias'.
'lias' after 'a' is 'ALIAS'.
(Other definitions for alias that I've seen before include "Otherwise known as" , "another name" , "False identity" , "False or assumed name" , "Handle that isn't genuine" .)