As an afterthought, they bite! (4)
I believe the answer is:
asps
'they bite' is the definition.
Although both the answer and 'bite' are nouns, they are not in the same form of the noun.
'as an afterthought' is the wordplay.
'an afterthought' becomes 'ps' (ps stands for postscript).
'as'+'ps'='ASPS'
Can you help me to learn more?
(Other definitions for asps that I've seen before include "Small venomous snakes" , "poisonous things" , "Poplars" , "Poisonous creepers" , "people" .)