The most common and most likely answer to this crossword clue is the 8 letter word TIRESOME
I've seen this crossword clue in The Times
T
I
R
E
S
O
M
E
More clues leading to the result TIRESOME
- Heavy book about anger, small and boring
- Irritating irregular rise in volume
- Boring father, first to last immersed in work
- Dull book about rise of anagram maker
- Boring book about passion ultimately sucks
- Remote is broken — it’s annoying
- Annoying failure first covered up in magazine
- Abnormal rise in volume that’s annoying
- Old timer’s rambling anecdote ultimately is boring
- Boring book about anger on top of softbacks
Buddy explains!

The straight part
We know that "Dull" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dull = tiresome.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 TIRESOME mean?
TIRESOME adjective- Causing fatigue or boredom; wearisome.