Rescue operation with lake drained to get beast (6)
I believe the answer is:
savage
'beast' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for savage that I've seen before include "Menacing, uncivilised" , "Brutal" , "Brutally criticise" , "Uncivilised and fierce" , "Uncivilised, vicious" .)