Missile part thinned with file (5)
I believe the answer is:
arrow
'missile' is the definition.
(arrow is a kind of missile)
'part thinned with file' is the wordplay.
'part thinned' becomes 'ar' (I can't explain this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'file' becomes 'row' (a file is a row of squares e.g. on a chess board).
'ar'+'row'='ARROW'
Can you help me to learn more?
(Other definitions for arrow that I've seen before include "Move swiftly" , "Dart; pointer" , "Mark to indicate direction" , "Archery projectile" , "production of Fletcher?" .)