It's easy to put letters in a file (6)
I believe the answer is:
facile
'it's easy' is the definition.
(synonyms)
'put letters in a file' is the wordplay.
'put letters' becomes 'ac' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'ac' put inside 'file' is 'FACILE'.
'to' is the 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" .)