Old cat, nine, exploded (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'cat nine exploded' is the wordplay.
'exploded' indicates anagramming the letters.
'cat'+'nine'='catnine'
'catnine' anagrammed gives 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Of times long past" , "so old-fashioned" , "From the distant past" , "no longer current" , "from Aristophanes' time" .)