Lay hold of key file (5)
I believe the answer is:
grasp
'lay hold of' is the definition.
(I've seen this before)
'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 "Appreciation" , "Support; rugby player" , "Follow - understand - seize" , "Firmly grip" , "Keep tight hold" .)