The most common and most likely answer to this crossword clue is the 11 letter word PALINDROMIC
I've seen this crossword clue in The Times
P
A
L
I
N
D
R
O
M
I
C
More clues leading to the result PALINDROMIC
Buddy explains!
The straight part
We know that "Description of mum, dad, nan and sis, in a word" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case description of mum, dad, nan and sis, in a word = palindromic.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.