Boiling could turn one soft (6)
I believe the answer is:
turnip
'boiling' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'turn one soft' is the wordplay.
'one' becomes 'i' (roman numeral).
'soft' becomes 'p' (musical abbreviation for 'piano').
'turn'+'i'+'p'='TURNIP'
'could' acts as a link.
Can you help me to learn more?
(Other definitions for turnip that I've seen before include "Fleshy edible white or yellow root" , "Watch" , "what you eat?" , "food item" , "Root vegetable of the mustard family" .)