Domino effect (5,8)
I believe the answer is:
chain reaction
'domino effect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chain reaction that I've seen before include "Series of events each caused by the previous one" , "that could get out of control" , "This process should continue" , "Continuing process" , "Series of stimulated responses" .)