Mark one stuck in corner? (5)
I believe the answer is:
stamp
'mark one stuck in corner?' is the definition.
'stamp' can be an answer for 'mark' (I have seen 'Imprinted mark' mean 'stamp' so perhaps 'mark' could also mean 'stamp'). I'm not sure about the 'one stuck in corner?' bit.
'mark one stuck in corner?' is the wordplay.
'mark' becomes 'm'.
'one stuck in' means one lot of letters goes inside another.
'corner?' becomes 'stap' (I can't justify this - if you can you should give a lot more credence to this answer).
'm' placed within 'stap' is 'STAMP'.
Can you help me to learn more?
(Other definitions for stamp that I've seen before include "Imprint, brand" , "Tread noisily" , "Validation for a letter - tread heavily" , "cast?" , "Imprinted mark" .)