It helps in making booze when put in still (5)
I believe the answer is:
yeast
'it helps in making booze' is the definition.
The definition suggests a singular noun which matches the answer.
'when put in still' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'put in' indicates putting letters inside.
'still' becomes 'yet' (yet can mean still or so far).
'as' put into 'yet' is 'YEAST'.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "fermenter" , "Brewing agent" , "lifting agent" , "Brewer" , "Fungus used in brewing and baking" .)