Plain sandwiches almost lasting until tomorrow (9)
I believe the answer is:
overnight
'until tomorrow' is the definition.
(I've seen this before)
'plain sandwiches almost' is the wordplay.
'plain' becomes 'overt' (I've seen this before).
'sandwiches' means one lot of letters goes inside another (to sandwich is to block something in).
'almost' becomes 'nigh' (synonyms).
'overt' placed around 'nigh' is 'OVERNIGHT'.
'lasting' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for overnight that I've seen before include "Until morning; suddenly" , "sudden" , "Extending from dusk to dawn" , "Lasting from dusk to dawn" , "until tomorrow" .)