The most common and most likely answer to this crossword clue is the 9 plus 3 letter word INVISIBLE INK
I
N
V
I
S
I
B
L
E
I
N
K
Buddy explains!
The straight part
We know that "it’s hard to read" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s hard to read = invisible ink.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.