The most common and most likely answer to this crossword clue is the 12 letter word EMBITTERMENT
I've seen this crossword clue in The Times
E
M
B
I
T
T
E
R
M
E
N
T
More clues leading to the result EMBITTERMENT
Buddy explains!
The straight part
We know that "Resentful state" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case resentful state = embitterment.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.