So hedge built to keep in initially pugnacious farm animal (8)
I believe the answer is:
sheepdog
'farm animal' is the definition.
(type of farm animal)
'so hedge built to keep in initially pugnacious' is the wordplay.
'built' is an anagram indicator.
'to keep in' is an insertion indicator.
'initially' says to take the initial letters.
The first letter of 'pugnacious' is 'p'.
'so'+'hedge'='sohedge'
'sohedge' anagrammed gives 'sheedog'.
'sheedog' placed around 'p' is 'SHEEPDOG'.
(Other definitions for sheepdog that I've seen before include "Shaggy-haired canine" , "Collie, for example" , "Pastoral assistant?" , "One drives over fields" , "Collie, perhaps" .)