Alcohol in plenty about to be drunk up (6)
I believe the answer is:
sterol
'alcohol' is the definition.
(I know that sterol is a type of alcohol)
'plenty about to be drunk up' is the wordplay.
'plenty' becomes 'lots' (lots of something).
'about' becomes 're' ('re' means regarding or about).
'to be drunk' indicates putting letters inside (inserted letters are drunk in).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'lots' placed around 're' is 'lorets'.
'lorets' backwards is 'STEROL'.
'in' acts as a link.
(Other definitions for sterol that I've seen before include "Solid form of alcohol" , "Ostler (anag) - natural alcohol" , "Ostler (anag.); an organic molecule" , "A solid higher alcohol" , "substance that doesn't dissolve" .)