Shiftily, maybe, disposing of a letter in a suspicious way (7)
I believe the answer is:
'shiftily maybe disposing of a letter' is the definition.
I can't tell whether this defines the answer.
'in a suspicious way' is the wordplay.
'in' indicates putting letters inside.
'a suspicious' becomes 'fishy' (I've seen this before).
'way' becomes 'il' (I can't justify this - if you can you should give a lot more credence to this answer).
'fishy' enclosing 'il' is 'FISHILY'.
Can you help me to learn more?