Large South American snake (3)
(Other definitions for boa that I've seen before include "... cold-blooded creature" , "Python" , "Non-venomous snake" , "strangler" , "feathers around the neck?" .)
'large south american snake' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for anaconda that I've seen before include "large reptile" , "A slippery customer" , "crawler" , "Large South American snake" , "Large water snake" .)