Caustic liquid left, yet unfinished (3)
I believe the answer is:
lye
'caustic liquid' is the definition.
(I've seen this before)
'left yet unfinished' is the wordplay.
'left' becomes 'l' (common abbreviation).
'unfinished' means to remove the last letter.
'yet' with its last letter removed is 'ye'.
'l'+'ye'='LYE'
(Other definitions for lye that I've seen before include "Strongly alkaline solution for washing or cleansing" , "Alkaline liquid" , "Alkali washing solution" , "Branch railway" , "Strongly alkaline wash" .)