Regard wise man we might hear in cold spell (3,3)
I believe the answer is:
ice age
'cold spell' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
I also know that 'cold' relates to this answer.
I don't understand how the remainder of the clue works.
(Other definitions for ice age that I've seen before include "Period of glaciation" , "Epoch of glaciers" , "Very cold era of earth's evolution" , "Glacial era" , "extreme climate" .)