The most common and most likely answer to this crossword clue is the 6 plus 5 letter word DIDDLY SQUAT
I've seen this crossword clue in The Guardian
D
I
D
D
L
Y
S
Q
U
A
T
More clues leading to the result DIDDLY SQUAT
Buddy explains!

The straight part
We know that "FA" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fa = diddly squat.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.