The most common and most likely answer to this crossword clue is the 10 letter word ILLITERACY
I've seen this crossword clue in The Guardian
I
L
L
I
T
E
R
A
C
Y
More clues leading to the result ILLITERACY
Buddy explains!

The straight part
We know that "Inability to grasp" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case inability to grasp = illiteracy.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 ILLITERACY mean?
ILLITERACY noun- The inability to read and write.
- The portion of a population unable to read and write, generally given as a percentage.
- A word, phrase or grammatical turn thought to be characteristic of an illiterate person.
- (by extension) Unlearnedness; the state of being ignorant or unlettered.