Ice mass (4)
I believe the answer is:
berg
'ice mass' is the definition.
(berg is a kind of ice mass)
This is all the clue.
(Other definitions for berg that I've seen before include "Lump of ice" , "He composed" , "feature of icy waters" , "Alban ----, composer of the incomplete opera 'Lulu' (1935)" , "ice in the drink" .)