The most common and most likely answer to this crossword clue is the 5 letter word BUTTY
I've seen this crossword clue in The Times
B
U
T
T
Y
More clues leading to the result BUTTY
Buddy explains!

The straight part
We know that "Sandwich" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sandwich = butty.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BUTTY mean?
BUTTY noun- A sandwich, usually with a hot savoury filling in a breadcake. The most common are chips, bacon, sausage and egg.
- Friend.
- A miner who works under contract, receiving a fixed amount per ton of coal or ore.
- A workmate.
- (among boys) A drudge; a cat's paw; someone who does the hard work; someone who is being taken advantage of by someone else.
- One of a pair of shoes or gloves.
- To work together; to keep company with.
- To cohabit; to reside with another as a couple.
- To act in concert with intent to defraud; to play unfairly.
- Resembling a heavy cart.