The most common and most likely answer to this crossword clue is the 6 letter word ABLAUT
I've seen this crossword clue in Financial Times
A
B
L
A
U
T
More clues leading to the result ABLAUT
Buddy explains!
The straight part
We know that "showing vowel gradation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing vowel gradation = ablaut.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 ABLAUT mean?
ABLAUT noun- The substitution of one root vowel for another, thus indicating a corresponding modification of use or meaning; vowel permutation; as, get and got; sing and song; hang and hung, distinct from the phonetic influence of a succeeding vowel.
- (of a vowel-containing linguistic component) To undergo a change of vowel.
- To cause to change a vowel.