The most common and most likely answer to this crossword clue is the 5 letter word SPELT
I've seen this crossword clue in The Guardian
S
P
E
L
T
Buddy explains!
The straight part
We know that "Rustic bread ingredient/specified in letters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rustic bread ingredient/specified in letters = spelt.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 SPELT mean?
SPELT verb- To read (something) as though letter by letter; to peruse slowly or with effort.
- (sometimes with “out”) To write or say the letters that form a word or part of a word.
- To be able to write or say the letters that form words.
- Of letters: to compose (a word).
- (with “out”) To clarify; to explain in detail.
- To indicate that (some event) will occur.
- To constitute; to measure.
- To speak, to declaim.
- To tell; to relate; to teach.
- To work in place of (someone).
- To rest (someone or something), to give someone or something a rest or break.
- To rest from work for a time.
- A grain, considered either a subspecies of wheat, Triticum aestivum subsp. spelta, or a separate species Triticum spelta or Triticum dicoccon.
- A thin piece of wood or metal; a splinter.
- Spelter.
- To split; to break; to spalt.