Hold a letter on file (5)
I believe the answer is:
grasp
'hold' is the definition.
(I know that hold can be written as grasp)
'a letter on file' is the wordplay.
'a letter' becomes 'g'.
'on' says to put letters next to each other.
'file' becomes 'rasp' (rasp is a kind of file).
'g'+'rasp'='GRASP'
(Other definitions for grasp that I've seen before include "Get hold of firmly" , "Understand (the point)" , "Understand; take in hand" , "Hold - understanding" , "Lay hold of" .)