Grip a good file (5)
I believe the answer is:
grasp
'grip' is the definition.
(synonyms)
'a good file' is the wordplay.
'a good' becomes 'g' (abbreviation).
'file' becomes 'rasp' (rasp is a kind of file).
'g'+'rasp'='GRASP'
(Other definitions for grasp that I've seen before include "Clutch" , "Catch on" , "Support; rugby player" , "Understand; take in hand" , "Hold tightly" .)