Kitten will act up (3)
I believe the answer is:
cat
'kitten' is the definition.
(thesaurus)
'act up' is the wordplay.
'up' indicates an anagram (letters get chucked up).
'act' anagrammed gives 'CAT'.
'will' is the link.
(Other definitions for cat that I've seen before include "Feline mammal" , "Sailor's whip with nine knotted strands" , "Tabby" , "Animal (domesticated or wild)" , "Common pet" .)