Mass is reduced by fusion (3)
I believe the answer is:
mix
'fusion' is the definition.
(thesaurus)
'mass is reduced by' is the wordplay.
'is reduced' indicates taking the first letters (the word is reduced to its first letter).
'by' becomes 'ix' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'mass' is 'm'.
'm'+'ix'='MIX'
Can you help me to learn more?
(Other definitions for mix that I've seen before include "Get on" , "= 1,009 (L.)" , "'Mingle, combine (3)'" , "Combine into one" , "Amalgamate, blend" .)