Back in grim lab, mentor will use formaldehyde to preserve it (6)
I believe the answer is:
embalm
'use formaldehyde to preserve it' is the definition.
I can't judge whether this defines the answer.
'back in grim lab mentor' is the wordplay.
'back' is a reversal indicator.
'in' indicates a hidden word.
'mlabme' is hidden within 'grim lab mentor'.
'mlabme' written backwards gives 'EMBALM'.
'will' acts as a link.
Can you help me to learn more?
(Other definitions for embalm that I've seen before include "Treat corpse so as to retard decay" , "Prepare for burial" , "Preserve body, mummify" , "Preserve corpse from decay" , "Use chemicals to preserve a dead body from decay" .)