The most common and most likely answer to this crossword clue is the 10 letter word NON-LINEAR
I've seen this crossword clue in The Guardian
N
O
N
-
L
I
N
E
A
R
Buddy explains!
The straight part
We know that "disproportionate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case disproportionate = non-linear.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 NON-LINEAR mean?
NON-LINEAR adjective- (of a set of points) Not lying on a straight line.
- (of a molecule) Whose atoms do not lie in a straight line.
- (of a function) Having a product of independent variables, or a variable with an exponent not equal to one.
- (of a system) Whose output is not directly proportional to its input.
- Erratic and unpredictable; tending to jump back and forth.