Animal killer in ridiculous energy-wasting operations (3-6)
I believe the answer is:
rat-poison
'animal killer' is the definition.
(rat poison kills rats)
'ridiculous energy-wasting operations' is the wordplay.
'ridiculous' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'wasting' indicates named letters should be taken away.
'operations' with 'e' taken out is 'oprations'.
'oprations' is an anagram of 'RAT-POISON'.
'in' is the link.
(Other definitions for rat-poison that I've seen before include "Rodent killer" , "Rodenticide" , "Vermin killer" .)