Cryptic formula for caustic soda in use, last of all (6)
I believe the answer is:
arcane
'cryptic' is the definition.
(I've seen this in another clue)
'formula for caustic soda in use last of all' is the wordplay.
'last of all' says to take the final letters.
The last letters of 'formula for caustic soda in use' are 'ARCANE'.
(Other definitions for arcane that I've seen before include "Recondite" , "Esoteric, mysterious" , "cryptic" , "deep" , "Ace ran, being mysterious" .)