Swine dropping back for snake (3)
I believe the answer is:
boa
'snake' is the definition.
(I know that boa is a type of constrictor)
'swine dropping back' is the wordplay.
'swine' becomes 'boar' (male pig).
'dropping back' means to remove the last letter.
'boar' with its final letter taken away is 'BOA'.
'for' acts as a link.
(Other definitions for boa that I've seen before include "strangler" , "feathers around the neck?" , "Snake; fur" , "Python" , "Large constrictor snake" .)