Speculator, denying role in collapse, causes trouble (4,2)
I believe the answer is:
acts up
'causes trouble' is the definition.
(act up can mean to misbehave)
'speculator denying role in collapse' is the wordplay.
'denying' indicates named letters should be taken away.
'in collapse' is an anagram indicator.
'speculator' with 'role' taken away is 'spcuat'.
'spcuat' is an anagram of 'ACTS UP'.
(Other definitions for acts up that I've seen before include "Behaves badly (colloq.)" , "Fails to function properly or behaves badly" , "Carries on" , "Causes trouble or misbehaves" , "Misbehaves badly" .)