Dissertation on case of security and origin of nuclear fusion (9)
I believe the answer is:
synthesis
'fusion' is the definition.
(I've seen this before)
'dissertation on case of security and origin of nuclear' is the wordplay.
'dissertation' becomes 'thesis' (synonyms).
'on' means one lot of letters go next to another.
'case of' means to remove the middle letters (outsides of).
'and' says to put letters next to each other.
'origin of' suggests taking the first letters.
'security' with its centre taken out is 'sy'.
The initial letter of 'nuclear' is 'n'.
'sy'+'n'='syn'
'thesis' put after 'syn' is 'SYNTHESIS'.
(Other definitions for synthesis that I've seen before include "Chemical or dialectical blending process" , "getting together tricky" , "In shy sets (anag.)" , "The making of compounds from simpler ones" , "Combination of different elements into a whole" .)