Period of glaciation (3,3)
I believe the answer is:
ice age
'period of glaciation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ice age that I've seen before include "Period of glaciation" , "Very cold era of earth's evolution" , "Glaciated period" , "Period of extreme cold" , "Cold spell (a very long one!)" .)