The most common and most likely answer to this crossword clue is the 6 letter word SALONS
I've seen this crossword clue in Financial Times
S
A
L
O
N
S
Buddy explains!

The straight part
We know that "spa centres or lounges in 11" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spa centres or lounges in 11 = salons.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 SALONS mean?
SALONS noun- A large room, especially one used to receive and entertain guests.
- A gathering of people for a social or intellectual meeting.
- An art gallery or exhibition; especially the Paris salon or autumn salon.
- A beauty salon or similar establishment.