Dog is company — right good one (5)
I believe the answer is:
corgi
'dog' is the definition.
(corgi is a kind of dog)
'company right good one' is the wordplay.
'company' becomes 'co' (abbreviation).
'right' becomes 'r' (abbreviation).
'good' becomes 'g' (abbreviation).
'one' becomes 'i' (Roman numeral).
'co'+'r'+'g'+'i'='CORGI'
'is' acts as a link.
(Other definitions for corgi that I've seen before include "Dog popular with British royals" , "One of the Queen's dogs?" , "Dog of Welsh breed, royal favourite" , "Queen's dog?" , "Royal canine" .)