Excellent environment for the Spanish cat (6)
I believe the answer is:
feline
'cat' is the definition.
(I know that cat can be written as feline)
'excellent environment for the spanish' is the wordplay.
'excellent' becomes 'fine' (synonyms).
'environment for' means one lot of letters goes inside another.
'the spanish' becomes 'el' ('the' in Spanish).
'fine' placed around 'el' is 'FELINE'.
(Other definitions for feline that I've seen before include "Relating to cats" , "(Relating to a) cat" , "Mog" , "An animal of the cat family" , "16 [TOM] perhaps" .)