Compound such as lime (6)
I believe the answer is:
alkali
'compound such as lime' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alkali that I've seen before include "Substance such as lime or caustic soda" , "Substance turning litmus paper blue" , "Acid neutraliser" , "Soluble compound that turns litmus paper blue" , "Basic stuff" .)