Derision at enormous return on energy (4)
I believe the answer is:
gibe
'derision' is the definition.
(thesaurus)
'enormous return on energy' is the wordplay.
'enormous' becomes 'big' ('big' can be similar in meaning to 'enormous').
'return' shows that the letters should be reversed in order.
'on' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'big' written backwards gives 'gib'.
'gib'+'e'='GIBE'
'at' acts as a link.
(Other definitions for gibe that I've seen before include "Hurtful remark" , "Barb, dig" , "Scoff (at) - taunt" , "Mocking remark" , "Jeer, mock" .)