Company, extremely loyal, that is dog (6)
I believe the answer is:
collie
'dog' is the definition.
(collie is a kind of dog)
'company extremely loyal that is' is the wordplay.
'company' becomes 'co' (abbreviation).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'loyal' with its centre removed is 'll'.
'co'+'ll'+'ie'='COLLIE'
(Other definitions for collie that I've seen before include "Farm dog" , "Popular sheepdog like Lassie" , "Long-haired intelligent dog" , "See 2" , "Scottish sheep dog" .)