The most common and most likely answer to this crossword clue is the 9 letter word EXECRABLE
I've seen this crossword clue in The Times
E
X
E
C
R
A
B
L
E
More clues leading to the result EXECRABLE
- Abhorrent former City mob dismissing head of bank
- River crustacean larvae, extremely vile
- River crustacean extremely loathsome and horrible
- Abhorrent mob half-heartedly supporting administrator
- Dreadful crustacean in river drained lake
- Awful old English king secured by rope
- Appalling manager somewhat disheartened crowd
- Awful from City playing b***** Real
- Early start. Cereal box empty. Disastrous. Awful
- Awful former king having the means to besiege the city
Buddy explains!

The straight part
We know that "Dreadful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dreadful = execrable.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 EXECRABLE mean?
EXECRABLE adjective- Of the poorest quality.
- Hateful.