Vast amount of hard water ruined beer in one hundred gallons (7)
I believe the answer is:
iceberg
'vast amount of hard water' is the definition.
I can't tell whether this defines the answer.
'ruined beer in one hundred gallons' is the wordplay.
'ruined' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'hundred' becomes 'c' (Roman numeral).
'gallons' becomes 'G' (abbreviation).
'beer' is an anagram of 'eber'.
'i'+'c'+'g'='icg'
'eber' placed within 'icg' is 'ICEBERG'.
Can you help me to learn more?
(Other definitions for iceberg that I've seen before include "Huge mass of frozen water" , "Large block of frozen water" , "unemotional person" , "Large floating mass of frozen water" , "Unlikely feature of Cos" .)