The most common and most likely answer to this crossword clue is the 7 letter word CAPITAL
I've seen this crossword clue in The Times
C
A
P
I
T
A
L
More clues leading to the result CAPITAL
Buddy explains!
The straight part
We know that "First-class/sort of letter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first-class/sort of letter = capital.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 CAPITAL mean?
CAPITAL noun- Already-produced durable goods available for use as a factor of production, such as steam shovels (equipment) and office buildings (structures).
- Money and wealth. The means to acquire goods and services, especially in a non-barter system.
- A city designated as a legislative seat by the government or some other authority, often the city in which the government is located; otherwise the most important city within a country or a subdivision of it.
- The most important city in the field specified.
- An uppercase letter.
- The uppermost part of a column.
- Knowledge; awareness; proficiency.
- (by extension) The chief or most important thing.
- Of prime importance.
- Chief, in a political sense, as being the seat of the general government of a state or nation.
- Excellent.
- Involving punishment by death.
- Uppercase.
- Of or relating to the head.