Fit as a lithe cat might be (8)
I believe the answer is:
athletic
'fit' is the definition.
(I've seen this before)
'a lithe cat might be' is the wordplay.
'might be' indicates anagramming the letters.
'lithe'+'cat'='lithecat'
'lithecat' anagrammed gives 'ATHLETIC'.
'as' acts as a link.
(Other definitions for athletic that I've seen before include "See 12" , "Active and fit for sport etc" , "Physically fit, strong and active" , "Wigan, for example" , "Lithe act (anag)" .)