Easy to slip bill into folder (6)
I believe the answer is:
facile
'easy' is the definition.
(synonyms)
'bill into folder' is the wordplay.
'bill' becomes 'ac' (short for account).
'into' means one lot of letters goes inside another.
'folder' becomes 'file folder' (file folder is a kind of folder).
'ac' placed into 'file' is 'FACILE'.
'to slip' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for facile that I've seen before include "Easy but glib" , "Arrived at without due care or effort" , "Easily achieved but of little value" , "Easy, simple" , "Superficial or too easily achieved" .)