Cold water ultimately softens wrinkles (7)
I believe the answer is:
creases
'wrinkles' is the definition.
('crease' can be a synonym of 'wrinkle')
'cold water ultimately softens' is the wordplay.
'cold' becomes 'c' (eg on taps).
'ultimately' indicates one should take the final letters.
'softens' becomes 'eases' (I've seen this before).
The final letter of 'water' is 'r'.
'c'+'r'+'eases'='CREASES'
(Other definitions for creases that I've seen before include "Pressed lines or crinkles" , "Cricket lines" .)