Insect beginning to worry snake (4)
I believe the answer is:
wasp
'insect' is the definition.
(wasp is a kind of insect)
'beginning to worry snake' is the wordplay.
'beginning to' indicates taking the first letters.
'snake' becomes 'asp' (asp is a kind of snake).
The first letter of 'worry' is 'w'.
'w'+'asp'='WASP'
(Other definitions for wasp that I've seen before include "Striped insect" , "Buzzer" , "Insect with a sting in its tail" , "White Anglo-Saxon Protestant" , "colonist" .)