Sign for the thesaurus he's left (6)
I believe the answer is:
taurus
'sign' is the definition.
(I know that Taurus is a sign)
'thesaurus he's left' is the wordplay.
'left' suggests deleting specific letters (some letters have left).
'thesaurus' with 'hes' removed is 'TAURUS'.
'for the' acts as a link.
(Other definitions for taurus that I've seen before include "Sign of the Bull from 12 across" , "April/May sign" , "Second sign of the zodiac, the Bull" , "Zodiacal bull" , "A group of stars" .)