The most common and most likely answer to this crossword clue is the 11 letter word ELASTICISED
I've seen this crossword clue in The Guardian
E
L
A
S
T
I
C
I
S
E
D
Buddy explains!
The straight part
We know that "built to be stretched" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case built to be stretched = elasticised.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 ELASTICISED mean?
ELASTICISED verb- To make with elastic, by attaching elastic bands, so it adjusts in size and remains snug. Usually done to clothing.