Two insects admit horse to be a huge creature (8)
I believe the answer is:
behemoth
'a huge creature' is the definition.
(I've seen this before)
'two insects admit 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).
'admit' is an insertion indicator.
'horse' becomes 'H' (abbreviation as in horse power).
'bee'+'moth'='beemoth'
'beemoth' placed around 'h' is 'BEHEMOTH'.
'to be' acts as a link.
(Other definitions for behemoth that I've seen before include "Huge animal as from the Bible" , "Anything abnormally large and powerful" , "Abnormally large creature" , "Monstrous beast (Job)" , "Something abnormally large and powerful" .)