Insect initially sucks croup on horse (6)
I believe the answer is:
scarab
'insect' is the definition.
(I've seen this before)
'initially sucks croup on horse' is the wordplay.
'initially' suggests taking the first letters.
'on' means one lot of letters go next to another.
'horse' becomes 'arab' (Arab is an example**).
The first letters of 'sucks croup' is 'sc'.
'sc'+'arab'='SCARAB'
(Other definitions for scarab that I've seen before include "Ancient Egyptian symbol" , "Sacred beetle of ancient Egypt" , "Large dung beetle" , "Large dark beetle" , "Beetle revered in ancient Egypt" .)