Sap distressed reptile (3)
I believe the answer is:
asp
'reptile' is the definition.
(asp is a kind of reptile)
'sap distressed' is the wordplay.
'distressed' indicates anagramming the letters.
'sap' anagrammed gives 'ASP'.
(Other definitions for asp that I've seen before include "Egyptian cobra" , "Cleopatra's poisoner" , "Venomous snake" , "Cold-blooded killer" , "One secreting poison" .)