Cat in silly prank (5)
I believe the answer is:
antic
'prank' is the definition.
(synonyms)
'cat in silly' is the wordplay.
'silly' is an anagram indicator.
'cat'+'in'='catin'
'catin' is an anagram of 'ANTIC'.
(Other definitions for antic that I've seen before include "Caper; grotesque (arch.)" , "Ludicrous act done for fun" , "Prank" , "Grotesque, fantastic (archaic)" , "seen to be bizarre, at one time" .)