The most common and most likely answer to this crossword clue is the 9 letter word ALLOMORPH
I've seen this crossword clue in The Guardian
A
L
L
O
M
O
R
P
H
Buddy explains!
The straight part
We know that "linguistic unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case linguistic unit = allomorph.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 ALLOMORPH mean?
ALLOMORPH noun- Any of the different crystalline forms of a substance.
- Any of the different phonological representations of a morpheme.