The most common and most likely answer to this crossword clue is the 8 letter word READABLE
R
E
A
D
A
B
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E
Buddy explains!
The straight part
We know that "Hard to put down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hard to put down = readable.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 READABLE mean?
READABLE adjective- (of handwriting, print, etc) legible, possible to read or at least decipher
- Which can be read—i.e. accessed or played—by a certain technical type of device
- (of a book) enjoyable to read, of an acceptable stylistic quality or at least functionally composed