Fit as a lithe cat could be (8)
I believe the answer is:
athletic
'fit' is the definition.
(I've seen this in another clue)
'a lithe cat could be' is the wordplay.
'could be' indicates an anagram.
'lithe'+'cat'='lithecat'
'lithecat' is an anagram of 'ATHLETIC'.
'as' acts as a link.
(Other definitions for athletic that I've seen before include "Sporting, vigorous" , "Fit and strong - football club?" , "With a strong and well proportioned body" , "Good at physical games" , "Strong and sporty" .)