Current sheep cause trouble (3,2)
I believe the answer is:
act up
'cause trouble' is the definition.
(I've seen this before)
'current sheep' is the wordplay.
'current' becomes 'ac' (alternating current).
'sheep' becomes 'tup' (tup is a kind of sheep).
'ac'+'tup'='ACT UP'
(Other definitions for act up that I've seen before include "Give trouble, make a fuss" , "Informally, behave badly or cause trouble" , "Behave badly, give trouble" , "malfunction" , "Behave like a brat" .)