Heartless pussycat misbehaves and makes trouble (4,2)
I believe the answer is:
acts up
'makes trouble' is the definition.
(act up can mean to misbehave)
'heartless pussycat misbehaves' is the wordplay.
'heartless' means to remove the middle letters.
'misbehaves' is an anagram indicator (I've seen 'misbehaving' mean this).
'pussycat' with its centre taken out is 'puscat'.
'puscat' anagrammed gives 'ACTS UP'.
'and' is the link.
(Other definitions for acts up that I've seen before include "Malfunctions" , "Behaves in a troublesome way" , "Behaves badly (colloq.)" , "Fails to function properly or behaves badly" , "Carries on" .)