The use of exaggeration for effect (9)
I believe the answer is:
hyperbole
'the use of exaggeration for effect' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hyperbole that I've seen before include "Deliberately extravagant exaggeration" , "Exaggerated language, not meant literally" , "Exaggerated talk from boy helper" , "The use of exaggeration for effect" , "not to be taken literally" .)