The most common and most likely answer to this crossword clue is the 4 letter word OAKS
I've seen this crossword clue in The Daily Telegraph
O
A
K
S
Buddy explains!
The straight part
We know that "Classic/acorn producers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case classic/acorn producers = oaks.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 OAKS mean?
OAKS noun- A deciduous tree with distinctive deeply lobed leaves, acorns, and notably strong wood, typically of England and northeastern North America, included in genus Quercus.
- The wood of the oak.
- A rich brown colour, like that of oak wood.
- Any tree of the genus Quercus, in family Fagaceae.
- Any tree of other genera and species of trees resembling typical oaks of genus Quercus in some ways.
- The outer (lockable) door of a set of rooms in a college or similar institution. (Often in the phrase "to sport one's oak").
- The flavor of oak.
- A stakes race in which entry is restricted to 3-year-old fillies.