Comprehension of key file (5)
I believe the answer is:
grasp
'comprehension' is the definition.
(I've seen this before)
'key file' is the wordplay.
'key' becomes 'g' (musical key).
'file' becomes 'rasp' (rasp is a kind of file).
'g'+'rasp'='GRASP'
'of' acts as a link.
(Other definitions for grasp that I've seen before include "Get hold of firmly" , "Comprehension" , "Hold firmly; understand" , "Understand; take in hand" , "Support; rugby player" .)