Infusion from nuclear fission melts away (6)
I believe the answer is:
Here is my best explanation:
'melts away' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Perhaps there's an association between them I don't understand?
'infusion from nuclear fission' is the subsidiary indication.
'from' is a deletion indicator.
'nuclearfission' with 'infusion' taken away is 'clears'.
(Other definitions for clears that I've seen before include "Empties", "sells" and "Exonerates".)