Understand key file (5)
I believe the answer is:
grasp
'understand' is the definition.
(grasping is a kind of understanding)
'key file' is the wordplay.
'key' becomes 'g' (I've seen this in other clues).
'file' becomes 'rasp' (rasp is a kind of file).
'g'+'rasp'='GRASP'
(Other definitions for grasp that I've seen before include "Keep tight hold" , "Appreciation" , "Understand (the point)" , "Seize - comprehend" , "Understand; hold tight" .)