Clone energy in unstable particle (9)
I believe the answer is:
replicate
'clone' is the definition.
(thesaurus)
'energy in unstable particle' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' indicates putting letters inside.
'unstable' indicates anagramming the letters.
'particle' with letters rearranged gives 'rplicate'.
'e' placed into 'rplicate' is 'REPLICATE'.
(Other definitions for replicate that I've seen before include "Copy, reproduce" , "Make an exact copy of" , "Copy exactly" .)