Crossword Genius

Constant greeting in top style (9,4)

Ross

I believe the answer is:

permanent wave

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'top style' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'constant greeting' is the wordplay.
'constant' becomes 'permanent' (I've seen this before).
'greeting' becomes 'wave' (gesture used as a greeting).
'permanent'+'wave'='PERMANENT WAVE'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for permanent wave that I've seen before include "Hairstyle" , "hair style" , "Treatment for straight hair" .)

I've seen this clue in The Daily Express.
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