Iceberg is a little lower (4)
I believe the answer is:
calf
This clue is a double definition.
'iceberg' is the first definition.
(I've seen this before)
'little lower' is the second definition.
(little cow - 'low' can mean to make a mooing sound)
'is a' acts as a link.
(Other definitions for calf that I've seen before include "Muscular back of the shank" , "Young bovine animal" , "Baby cow" , "Young cow/bull" , "Young giraffe - piece of iceberg" .)