Outpouring of energy following nuclear reaction (8)
I believe the answer is:
effusion
'outpouring' is the definition.
(I know that effusion is a type of outpouring)
'energy following nuclear reaction' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'following' becomes 'f' (used when citing page numbers etc).
'nuclear reaction' becomes 'fusion' (fusion is a kind of nuclear reaction).
'e'+'f'+'fusion'='EFFUSION'
'of' acts as a link.
(Other definitions for effusion that I've seen before include "Unrestrained outpouring" , "A pouring forth of thought or feeling" , "Outburst" , "Gush" , "A streaming out" .)