The most common and most likely answer to this crossword clue is the 4 plus 2 letter word HANG UP
I've seen this crossword clue in Independent
H
A
N
G
U
P
More clues leading to the result HANG UP
Buddy explains!

The straight part
We know that "double defn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case double defn = hang up.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 HANG UP mean?
HANG UP noun- An emotional difficulty or a psychological inhibition; a complex.
- An unforeseen obstacle to progress; a hitch.