The most common and most likely answer to this crossword clue is the 9 letter word PLIMSOLLS
I've seen this crossword clue in The Guardian
P
L
I
M
S
O
L
L
S
More clues leading to the result PLIMSOLLS
Buddy explains!
The straight part
We know that "footwear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case footwear = plimsolls.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 PLIMSOLLS mean?
PLIMSOLLS noun- A rubber-soled lace-up canvas shoe for sports or onboard ships; a precursor of trainers.
- The plimsoll symbol ⦵ (or o) that is used as a superscript in the notation of thermodynamics to indicate an arbitrarily chosen non-zero reference point.