Squeezing snake (11)
'squeezing snake' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for boa that I've seen before include "Long, thin, feathery scarf" , "... cold-blooded creature" , "One who puts the squeeze on" , "feathers around the neck?" , "Potential killer" .)
(Other definitions for constrictor that I've seen before include "snake" , "One drawing together" .)