Understanding key to file (5)
I believe the answer is:
grasp
'understanding' is the definition.
(I know that grasp is a type of understanding)
'key to file' is the wordplay.
'key' becomes 'g' (musical key).
'to file' becomes 'rasp' (rasp is a kind of file).
'g'+'rasp'='GRASP'
(Other definitions for grasp that I've seen before include "Follow - understand - seize" , "Seize - comprehend" , "Take firm hold" , "Seize firmly - or understand (5)" , "Hold tightly" .)