In retrospect, research site divides Frenchmen initially eager to use preservatives in a deadly way (6)
I believe the answer is:
embalm
'to use preservatives in a deadly way' is the definition.
I can't judge whether this definition defines the answer.
'in retrospect research site divides frenchmen initially eager' is the wordplay.
'in retrospect' is a reversal indicator.
'research site' becomes 'lab' (I've seen this in another clue).
'divides' is an insertion indicator (inserted letters divide the word).
'frenchmen' becomes 'mm' (two M's - M is short for Monsieur).
'initially' suggests taking the first letters.
The initial letter of 'eager' is 'e'.
'mm'+'e'='mme'
'lab' placed within 'mme' is 'mlabme'.
'mlabme' back-to-front is 'EMBALM'.
Can you help me to learn more?
(Other definitions for embalm that I've seen before include "Preserve from decay in death with chemicals" , "Make into mummy" , "Prepare for burial" , "Preserve body, mummify" , "Treat corpse so as to retard decay" .)