Act in a certain way (6)
I believe the answer is:
behave
'act in a certain way' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for behave that I've seen before include "Act in a specified way" , "Comport oneself" , "Mind your manners" , "Act in a certain way" , "Conduct oneself properly" .)