Snake's adversary -- Saint Patrick, originally (3)
I believe the answer is:
asp
'snake's' is the definition.
(asp is a kind of snake)
'adversary saint patrick originally' is the wordplay.
'originally' suggests taking the first letters.
The initial letters of 'adversary saint patrick' is 'ASP'.
(Other definitions for asp that I've seen before include "Snake, killed Cleopatra" , "Snake seen in a spa, strangely" , "Joe Blake is Australian species" , "Snake found in spa" , "snake appears" .)