Those drinking slowly devour litres like mules (8)
I believe the answer is:
slippers
'like mules' is the definition.
(footwear similar to mules)
'those drinking slowly devour litres' is the wordplay.
'those drinking slowly' becomes 'sippers' (to sip is to drink slowly).
'devour' is an insertion indicator (inserted letters are swallowed or devoured).
'litres' becomes 'l'.
'sippers' placed around 'l' is 'SLIPPERS'.
(Other definitions for slippers that I've seen before include "Indoor shoes" , "Punishes" , "CLOTHING" , "those who fall over?" , "such may be discarded by bed" .)