No-cost energy when very cold (8)
I believe the answer is:
freezing
'very cold' is the definition.
(I've seen this before)
'no-cost energy' is the wordplay.
'no cost' becomes 'free' (I've seen this in other clues).
'energy' becomes 'zing' (I've seen this before**).
'free'+'zing'='FREEZING'
'when' acts as a link.
(Other definitions for freezing that I've seen before include "Becoming ice" , "Like ice" , "Changing to ice" , "suffering from cold" , "Ice-cold" .)