Sign that's plain by road regularly ignored (5)
I believe the answer is:
libra
'sign' is the definition.
(sign of the zodiac)
'plain by road regularly ignored' is the wordplay.
'regularly ignored' indicates alternate letters (letters removed at regular intervals).
'plain'+'by'+'road'='plainbyroad'
The alternate letters of 'plainbyroad' are 'LIBRA'.
'that's' is the link.
(Other definitions for libra that I've seen before include "Person born under Scales sign" , "Scales constellation" , "Constellation near Virgo" , "Balance constellation" , "Scales star sign" .)