Cryptic crossword clue
Regret James turning up second at one’s party 15

The most common and most likely answer to this crossword clue is the 15 letter word DISSATISFACTION

I've seen this crossword clue in The Daily Telegraph
D
I
S
S
A
T
I
S
F
A
C
T
I
O
N

Buddy explains!

The straight part
We know that "Regret" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case regret = dissatisfaction.
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 DISSATISFACTION mean?

DISSATISFACTION noun
  1. Unhappiness or discontent
  2. The cause of such feelings

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