Betting system that may go to one's head (4)
I believe the answer is:
perm
'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" .)