It's easy to put letters in a file (6)
I believe the answer is:
facile
'it's easy' is the definition.
(synonyms)
'letters in a file' is the wordplay.
'letters' becomes 'ac' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'ac' going into 'file' is 'FACILE'.
'to put' acts as a link.
Can you help me to learn more?
(Other definitions for facile that I've seen before include "Oversimplified" , "Simplistic" , "Superficial - glib" , "Over-simple, superficial" , "Arrived at without due care or effort" .)