Wave combination in pools (4)
I believe the answer is:
perm
I believe this is a double definition.
'wave' is the first definition.
(perming is a kind of waving)
'combination in pools' is the second definition.
(short for permutation in football pools)
(Other definitions for perm that I've seen before include "combo in pools" , "A hairdo; Urals city" , "Hairstyle treatment" , "Hair style (abbr)" , "Hair setting method" .)