Potion once thought to prolong life (6)
I believe the answer is:
elixir
'potion once thought to prolong life' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for elixir that I've seen before include "Miraculous cure" , "Magic potion" , "Magical potion to prolong life" , "Substance that cures all ills?" , "Substance believed to cure all ills" .)