The most common and most likely answer to this crossword clue is the 7 letter word CURLING
I've seen this crossword clue in The Times
C
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Buddy explains!
The straight part
We know that "hair treatment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hair treatment = curling.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 CURLING mean?
CURLING verb- To cause to move in a curve.
- To make into a curl or spiral.
- To assume the shape of a curl or spiral.
- To move in curves.
- To take part in the sport of curling.
- To exercise by bending the arm, wrist, or leg on the exertion against resistance, especially of the biceps.
- To twist or form (the hair, etc.) into ringlets.
- To deck with, or as if with, curls; to ornament.
- To raise in waves or undulations; to ripple.
- (hat-making) To shape (the brim of a hat) into a curve.
- The action or motion of something that curls or is curled.
- A winter sport where players aim and slide stones down a sheet of ice and attempt to get their color stones closest to the house (a circular target marked on the ice).