Deadly snakes (4)
(Other definitions for boas that I've seen before include "Long fluffy scarves of feathers" , "Cold-blooded creatures" , "Large snakes" , "21 [SNAKES]" .)
'deadly snakes' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for asps that I've seen before include "poisonous things" , "native Africans?" , "Pass among snakes" , "Small venomous snakes" , "Cleopatra's cobras" .)