Become active (4)
I believe the answer is:
stir
'become active' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stir that I've seen before include "Act as an agitator" , "Thrill" , "Arouse (a memory)" , "A slangy prison - whisk it" , "Prison - mix" .)