Scorpio or Sagittarius (4)
I believe the answer is:
sign
'scorpio or sagittarius' is the definition.
(I know that Scorpio is a sign)
This is all the clue.
(Other definitions for sign that I've seen before include "Engage by written agreement" , "Indication - put one's name down" , "Communicate to the deaf" , "Leo, for instance" , "Augury" .)