The most common and most likely answer to this crossword clue is the 3 letter word GUY
I've seen this crossword clue in The Times
G
U
Y
More clues leading to the result GUY
Buddy explains!

The straight part
We know that "treatment centre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case treatment centre = guy.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 GUY mean?
GUY noun- An effigy of a man burned on a bonfire on the anniversary of the Gunpowder Plot (5th November).
- A person of eccentric appearance or dress; a "fright".
- A man, fellow.
- (especially in the plural) A person (see usage notes).
- (of animals and sometimes objects) Thing, creature.
- Thing, unit.
- (term of address) Buster, Mack, fella, bud, man.
- To exhibit an effigy of Guy Fawkes around the 5th November.
- To make fun of, to ridicule with wit or innuendo.
- To play in a comedic manner.
- A guide; a leader or conductor.
- A support rope or cable used to guide, steady or secure something which is being hoisted or lowered.
- A support to secure or steady something prone to shift its position or be carried away (e.g. the mast of a ship or a suspension-bridge).
- To equip with a support cable.