Sign of a tailless, venomous creature (7)
I believe the answer is:
scorpio
'sign' is the definition.
(sign of the zodiac)
'tailless venomous creature' is the wordplay.
'tailless' means to remove the last letter (the tail of the word is removed).
'venomous creature' becomes 'scorpion' (I've seen this before).
'scorpion' with its last letter taken off is 'SCORPIO'.
'of a' acts as a link.
(Other definitions for scorpio that I've seen before include "Sign of stinging arachnid" , "Astrological sign" , "Zodiac sign like mini-lobster" , "Eighth sign of the Zodiac, kind of spider" , "Zodiacal sign" .)