The most common and most likely answer to this crossword clue is the 5 letter word SHALT
I've seen this crossword clue in The Guardian
S
H
A
L
T
More clues leading to the result SHALT
Buddy explains!
The straight part
We know that "Must old-fashioned" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case must old-fashioned = shalt.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 SHALT mean?
SHALT verb- (modal, auxiliary verb, defective) Used before a verb to indicate the simple future tense in the first person singular or plural.
- Used similarly to indicate determination or obligation in the second and third persons singular or plural.
- Used in questions with the first person singular or plural to suggest a possible future action.
- To owe.