Ointment -- litres in reserve (5)
I believe the answer is:
salve
'ointment' is the definition.
('salve' can be a synonym of 'ointment')
'litres in reserve' is the wordplay.
'litres' becomes 'l'.
'in' means one lot of letters goes inside another.
'reserve' becomes 'save' (saving is a kind of reserving).
'l' put inside 'save' is 'SALVE'.
(Other definitions for salve that I've seen before include "Soothing remedy" , "Roman greeting" , "Soothing ointment - may also be applied to feelings" , "Skin ointment" , "Unguent" .)