Act on (6)
I believe the answer is:
affect
'act on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for affect that I've seen before include "Arouse sadness or sympathy like tear-jerker" , "Act upon" , "Have repercussions on" , "Influence; make pretence of" , "impress" .)