Two insects circling horse — huge beast (8)
I believe the answer is:
behemoth
'huge beast' is the definition.
'behemoth' can be an answer for 'beast' (I've seen this before). I'm not certain of the 'huge' bit.
'two insects circling horse' is the wordplay.
'two insects' can mean two replacements for 'insect'.
'insect' becomes 'bee' (bee is a type of insect).
'insect' becomes 'moth' (moth is a kind of insect).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'horse' becomes 'H' (abbreviation as in horse power).
'bee'+'moth'='beemoth'
'beemoth' enclosing 'h' is 'BEHEMOTH'.
Can you help me to learn more?
(Other definitions for behemoth that I've seen before include "Gigantic beast in Job" , "Great beast of the Bible" , "Huge organisation" , "Anything abnormally large and powerful" , "A huge beast in the Bible" .)