Take away a sign (5)
I believe the answer is:
minus
This clue is a double definition.
'take away' is the first definition.
(minus sign in mathematics)
'a sign' is the second definition.
(minus sign is a kind of sign)
(Other definitions for minus that I've seen before include "Less, without" , "Negative sign - negative thing" , "deficiency" , "'Take away' sign" , "Lacking; below zero" .)