The most common and most likely answer to this crossword clue is the 9 letter word AGAR-AGAR
I've seen this crossword clue in The Times
A
G
A
R
-
A
G
A
R
More clues leading to the result AGAR-AGAR
- Gelatinous substance produced by a fish more than once
- Repeatedly use a cloth over jelly
- Seaweed’s energy expended by a motor-home on a run
- A German with silver ra-ra skirts overturned jelly
- Setter noted patterns in reverse
- A swimmer a newspaper turned to jelly
- Turkish commander runs with a fish in jelly
- A sort of fish repeatedly put into gelatinous substance
- . Jelly in a bait brought up a fish
- A rag is returned by chief with money for some gelatinous substance
Buddy explains!

The straight part
We know that "Gelatinous substance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gelatinous substance = agar-agar.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 AGAR-AGAR mean?
AGAR-AGAR noun- Agar.