Big cat's junior imbibing water in Spain (6)
I believe the answer is:
jaguar
'big cat's' is the definition.
(jaguar is a kind of big cat)
'junior imbibing water in spain' is the wordplay.
'junior' becomes 'jr' ('Jr' can be a synonym of 'junior').
'imbibing' indicates putting letters inside.
'water in spain' becomes 'agua' ('water' in Spanish).
'jr' placed around 'agua' is 'JAGUAR'.
(Other definitions for jaguar that I've seen before include "Big cat; make of car" , "Powerful cat" , "Large spotted cat" , "Fast car or tropical American cat" , "Wildcat" .)