What produces reduction of scale in floods more frequently (5,8)
I believe the answer is:
water softener
'what produces reduction of scale' is the definition.
(product which reduces limescale)
'floods more frequently' is the wordplay.
'floods' becomes 'waters' (I've seen this before).
'more frequently' becomes 'oftener' ('oftener' can be a synonym of 'more frequently').
'waters'+'oftener'='WATER-SOFTENER'
'in' is the link.
(Other definitions for water softener that I've seen before include "Remover of hardness (eg, from mains supply)" , "Substance aiding the production of suds" , "it can facilitate laundering operation" .)