The most common and most likely answer to this crossword clue is the 5 plus 4 letter word FROZE OVER
I've seen this crossword clue in The Guardian
F
R
O
Z
E
O
V
E
R
Buddy explains!
The straight part
We know that "completed what the Earth did in the Ice Age" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case completed what the earth did in the ice age = froze over.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.