Understand something at last on file (5)
I believe the answer is:
grasp
'understand' is the definition.
(I know that understand can be written as grasp)
'something at last on file' is the wordplay.
'at last' suggests the final letters.
'on' says to put letters next to each other.
'file' becomes 'rasp' (rasp is a kind of file).
The final letter of 'something' is 'g'.
'g'+'rasp'='GRASP'
(Other definitions for grasp that I've seen before include "Lay hold of" , "Understand (the point)" , "Understand; hold tight" , "Keep tight hold" , "Seize - comprehend" .)