Firm getting two pounds that is for a dog (6)
I believe the answer is:
collie
'for a dog' is the definition.
(collie is a kind of dog)
'firm getting two pounds that is' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'getting' means one lot of letters go next to another.
'two' says to double the letters.
'pounds' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'l' duplicated is 'll'.
'co'+'ll'+'ie'='COLLIE'
(Other definitions for collie that I've seen before include "Type of sheepdog" , "Long-haired sheepdog" , "Popular sheepdog like Lassie" , "Breed of sheepdog" , "Popular kind of sheepdog" .)