Two insects biting horse, a whopper (8)
I believe the answer is:
behemoth
'a whopper' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'two insects biting horse' is the wordplay.
'two insects' can mean two substitutions for 'insect'.
'insect' becomes 'bee' (bee is a type of insect).
'insect' becomes 'moth' (moth is a kind of insect).
'biting' means one lot of letters goes inside another (some letters bite or swallow others).
'horse' becomes 'h' (synonyms).
'bee'+'moth'='beemoth'
'beemoth' placed around 'h' is 'BEHEMOTH'.
Can you help me to learn more?
(Other definitions for behemoth that I've seen before include "Great beast of the Bible" , "Gigantic beast in Job" , "Huge animal as from the Bible" , "Something of impressive size or power" , "big beastly type" .)