Crossword Genius

Betting system that may go to one's head (4)

Ross

I believe the answer is:

perm

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'head' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Maybe there's a link between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for perm that I've seen before include "Hair setting treatment; pools selection system" , "Hairstyle treatment" , "Wavy hairstyle" , "Possibility in pool" , "make waves" .)

I've seen this clue in the Evening Standard.
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