The most common and most likely answer to this crossword clue is the 1 plus 6 plus 5 letter word A PRETTY PENNY
I've seen this crossword clue in The Guardian
A
P
R
E
T
T
Y
P
E
N
N
Y
More clues leading to the result A PRETTY PENNY
Buddy explains!
The straight part
We know that "an arm and a leg /an arm and a leg" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an arm and a leg /an arm and a leg = a pretty penny.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.