Large-scale source of power in shifting ice (4)
I believe the answer is:
epic
'large-scale' is the definition.
(I've seen this before)
'source of power in shifting ice' is the wordplay.
'source of' indicates taking the first letters.
'in' indicates putting letters inside.
'shifting' indicates an anagram.
The first letter of 'power' is 'p'.
'ice' with letters rearranged gives 'eic'.
'p' inserted inside 'eic' is 'EPIC'.
(Other definitions for epic that I've seen before include "Long, heroic story" , "Long poem telling of great events" , "Exodus, say" , "Long literary work about great events" , "Long poem/story" .)