The most common and most likely answer to this crossword clue is the 16 letter word HOWS-YOUR-FATHER
I've seen this crossword clue in The Times
H
O
W
S
-
Y
O
U
R
-
F
A
T
H
E
R
Buddy explains!
The straight part
We know that "It" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it = hows-your-father.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.