The most common and most likely answer to this crossword clue is the 11 letter word EXPONENTIAL
E
X
P
O
N
E
N
T
I
A
L
More clues leading to the result EXPONENTIAL
Buddy explains!

The straight part
We know that "description of rapid rise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case description of rapid rise = exponential.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 EXPONENTIAL mean?
EXPONENTIAL noun- Any function that has an exponent as an independent variable.
- Relating to an exponent.
- Expressed in terms of a power of e.
- Characterised by a rate of change that is proportional to the value of the varying quantity, or, equivalently, by a doubling or halving over successive fixed intervals of time or other parameter.
- Characterised by a very rapid rate of change, especially increase.