The most common and most likely answer to this crossword clue is the 6 letter word SHRANK
I've seen this crossword clue in The Guardian
S
H
R
A
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Buddy explains!

The straight part
We know that "became smaller" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case became smaller = shrank.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 SHRANK mean?
SHRANK verb- To cause to become smaller.
- To become smaller; to contract.
- To cower or flinch.
- To draw back; to withdraw.
- To withdraw or retire, as from danger.
- To move back or away, especially because of fear or disgust.
- (Pennsylvania Dutch English) A large highly decorative German/Dutch-style piece of furniture, which combines aspects of a clothing wardrobe, curio, and cabinet.