Behaving in a certain way (6)
I believe the answer is:
acting
'behaving in a certain way' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for acting that I've seen before include "Performing or standing in for another" , "Performing the role of" , "Playing a part" , "in a provisional role" , "Taking on duties temporarily" .)