Very cold spell -- one bear's home, perhaps conserving energy (3,3)
I believe the answer is:
ice age
'very cold spell' is the definition.
(spell can mean a period of time)
'one bear's home perhaps conserving energy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'bear's home perhaps' becomes 'cage' (a bear might be kept in a cage).
'conserving' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cage' placed around 'e' is 'ceage'.
'i'+'ceage'='ICE AGE'
(Other definitions for ice age that I've seen before include "When glaciers covered a large part of the earth's surface" , "Period of glaciation" , "Freezing era" , "Very cold era of earth's evolution" , "Glacial epoch" .)