Clear up cat's mess? Splendid! (11)
I believe the answer is:
spectacular
'splendid' is the definition.
(I've seen this before)
'clear up cat's mess?' is the wordplay.
'mess?' is an anagram indicator (I've seen 'mess' mean this).
'clear'+'up'+'cats'='clearupcats'
'clearupcats' with letters rearranged gives 'SPECTACULAR'.
(Other definitions for spectacular that I've seen before include "Extravaganza" , "Sensational in appearance" , "Impressive, striking" , "Extremely impressive" , "Lavish production" .)