Snakes beginning to terrify crow (5)
I believe the answer is:
boast
'crow' is the definition.
(I know that crow is a is a more specific form of the action boast)
'snakes beginning to terrify' is the wordplay.
'snakes' becomes 'boas' (boa is a kind of snake).
'beginning to' suggests taking the first letters.
The first letter of 'terrify' is 't'.
'boas'+'t'='BOAST'
(Other definitions for boast that I've seen before include "Show off" , "Brag about - squash shot" , "Blow one's own trumpet" , "Expression of excessive pride" , "Crow, brag" .)