Fish tails in horse meat for dog (6)
I believe the answer is:
basset
'for dog' is the definition.
(type of dog)
'fish tails in horse meat' is the wordplay.
'fish' becomes 'bass' (bass is a kind of fish).
'tails in' says to take the final letters (I've seen 'tail in' mean this).
The final letters of 'horse meat' are 'et'.
'bass'+'et'='BASSET'
(Other definitions for basset that I've seen before include "Hare-hunting hound among beasts" , "Beasts (anag.)" , "Type of horn" , "Sturdy hunting dog" , "Short-legged hound that hunts hares" .)