Whip snake's first firm egg (7)
I believe the answer is:
scourge
'whip' is the definition.
(I know that scourge is a type of whip)
'snake's first firm egg' is the wordplay.
'snake's first' becomes 's' (1st letter of 'snake').
'firm' becomes 'co' (a firm is a company).
'egg' becomes 'urge' (I've seen this in another clue).
's'+'co'+'urge'='SCOURGE'
(Other definitions for scourge that I've seen before include "big nuisance" , "Whip; source of suffering" , "A cause of great trouble" , "Afflict" , "Plague; whip" .)