Two litres covered by one-time liquid softener (9)
I believe the answer is:
emollient
'softener' is the definition.
(an emollient softens the skin)
'two litres covered by one-time liquid' is the wordplay.
'two litres' can mean two replacements for 'litre'.
'litre' becomes 'L' (abbreviation).
'litre' becomes 'L' (abbreviation).
'covered by' means one lot of letters goes inside another.
'liquid' indicates anagramming the letters.
'l'+'l'='ll'
'one'+'time'='onetime'
'onetime' with letters rearranged gives 'emoient'.
'll' put inside 'emoient' is 'EMOLLIENT'.
(Other definitions for emollient that I've seen before include "attempting to avoid confrontation" , "cream" , "Ointment, salve" , "soothing agent" , "Conciliatory" .)