Large-scale source of power in melting ice (4)
I believe the answer is:
epic
'large-scale' is the definition.
(I've seen this before)
'source of power in melting ice' is the wordplay.
'source of' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'melting' indicates an anagram.
The initial letter of 'power' is 'p'.
'ice' is an anagram of 'eic'.
'p' put inside 'eic' is 'EPIC'.
(Other definitions for epic that I've seen before include "Long adventure story" , "Heroic; saga" , "On a grand scale, maybe a poem" , "Long poem telling of heroic events" , "Marathon" .)