Cat, lithe, ridiculously fit (8)
I believe the answer is:
athletic
'fit' is the definition.
(I've seen this in another clue)
'cat lithe ridiculously' is the wordplay.
'ridiculously' indicates an anagram.
'cat'+'lithe'='catlithe'
'catlithe' anagrammed gives 'ATHLETIC'.
(Other definitions for athletic that I've seen before include "Sporting, vigorous" , "Lusty" , "See 12" , "Strong and vigorous" , "Team" .)