Horse beginning to rear when meeting a snake (5)
I believe the answer is:
cobra
'snake' is the definition.
(cobra is a kind of snake)
'horse beginning to rear when meeting a' is the wordplay.
'horse' becomes 'cob' (type of horse).
'beginning to' suggests taking the first letters.
'when meeting' says to put letters next to each other.
The initial letter of 'rear' is 'r'.
'cob'+'r'+'a'='COBRA'
(Other definitions for cobra that I've seen before include "one leading charmed life?" , "Poisonous snake" , "venomous type" , "Deadly snake called King" , "Venomous hooded snake" .)