Certain snake (3)
I believe the answer is:
asp
'certain snake' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for asp that I've seen before include "Cold-blooded killer" , "A glider's what's called for" , "Snake seen in a spa, strangely" , "Snake of the Nile" , "Small poisonous snake" .)