The most common and most likely answer to this crossword clue is the 9 plus 4 letter word IDENTICAL TWIN
I've seen this crossword clue in Financial Times
I
D
E
N
T
I
C
A
L
T
W
I
N
More clues leading to the result IDENTICAL TWIN
- Double success after Latin edict redrafted
- Sibling success after Latin edict rubbished
- Smashing incidental wit shown by Dromio of Ephesus for example
- Double triumph led Titanic novel to come first
- 1 Across may be otherwise wanted in italics as absent
- A twit inclinedto double
- After first of illusions bamboozled, I’d then let cat out of the bag – and one of two peas!
Buddy explains!
The straight part
We know that "one of two peas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of two peas = identical twin.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.