Lithe cat unusually active (8)
I believe the answer is:
athletic
'active' is the definition.
(similar in meaning)
'lithe cat unusually' is the wordplay.
'unusually' indicates an anagram.
'lithe'+'cat'='lithecat'
'lithecat' is an anagram of 'ATHLETIC'.
(Other definitions for athletic that I've seen before include "Lusty" , "Fit and strong - football club?" , "Lithe act (anag)" , "Muscular" , "Strong and active, good at sports" .)