The most common and most likely answer to this crossword clue is the 8 letter word ACCRETED
I've seen this crossword clue in Independent
A
C
C
R
E
T
E
D
Buddy explains!
The straight part
We know that "built up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case built up = accreted.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 ACCRETED mean?
ACCRETED verb- To grow together, combine; to fuse.
- To adhere; to grow or to be added to gradually.
- To make adhere; to add; to make larger or more, as by growing.