Understand without opening file (4)
I believe the answer is:
rasp
'file' is the definition.
(I know that rasp is a type of file)
'understand without opening' is the wordplay.
'understand' becomes 'grasp' (grasping is a kind of understanding).
'without opening' means to remove the first letter.
'grasp' with its initial letter taken off is 'RASP'.
(Other definitions for rasp that I've seen before include "unpleasant noise" , "Make a grating noise" , "Smoother" , "Filing tool" , "Harsh noise - file" .)