The most common and most likely answer to this crossword clue is the 11 letter word UNNECESSARY
I've seen this crossword clue in The Daily Telegraph
U
N
N
E
C
E
S
S
A
R
Y
More clues leading to the result UNNECESSARY
Buddy explains!

The straight part
We know that "Superfluous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case superfluous = unnecessary.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.
What does UNNECESSARY mean?
UNNECESSARY adjective- Not needed or necessary.
- Done in addition to requirements; unrequired.