Sign name over two others, one of four famous ones (6)
I believe the answer is:
lennon
'others one of four famous ones' is the definition.
I can't tell whether this definition defines the answer.
'sign name over two' is the wordplay.
'sign' becomes 'leo' (star sign).
'name' becomes 'n' (common abbreviation e.g. on forms).
'over' indicates putting letters inside.
'two' becomes 'nn' (I am not sure about this - if you are sure you should believe this answer much more).
'leo'+'n'='leon'
'leon' enclosing 'nn' is 'LENNON'.
(Other definitions for lennon that I've seen before include "John - (Imagine)" , "assassinated pop star" , "Song writer" , "Famous Liverpudlian" , "John -, musician murdered nineteen eighty" .)