Momentous handle turns on wilful computer (7)
I believe the answer is:
epochal
'momentous' is the definition.
(thesaurus)
'handle turns on wilful computer' is the wordplay.
I cannot really understand how this works, but
'on' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'computer' could be 'pc' (PC is an example) and 'pc' is present in the leftover letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for epochal that I've seen before include "of a certain age" , "Of a period in history" , "beginning a new era" , "of a particular age" , "Like world-shaking events" .)