Cryptic crossword clue
Familiar rascal oddly hiding in nunnery 10

The most common and most likely answer to this crossword clue is the 10 letter word CONVERSANT

I've seen this crossword clue in The Daily Telegraph
C
O
N
V
E
R
S
A
N
T

Buddy explains!

The straight part
We know that "Familiar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case familiar = conversant.
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.

Crossword Answer definitions

CONVERSANT noun
  1. One who converses with another.
CONVERSANT adjective
  1. Closely familiar; current; having frequent interaction
  2. Familiar or acquainted by use or study; well-informed; versed
  3. Concerned; occupied.

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