The most common and most likely answer to this crossword clue is the 9 letter word ENLARGERS
I've seen this crossword clue in The Guardian
E
N
L
A
R
G
E
R
S
More clues leading to the result ENLARGERS
Buddy explains!
The straight part
We know that "They blow up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they blow up = enlargers.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 ENLARGERS mean?
ENLARGERS noun- Any device that makes something bigger, or makes it appear bigger.
- An optical device used to make enlarged prints from a photographic negative