Rubbish about litre being a very generous measure of spirits (6)
I believe the answer is:
triple
'a very generous measure of spirits' is the definition.
(triple shot of alcohol)
'rubbish about litre' is the wordplay.
'rubbish' becomes 'tripe' (both can mean nonsense).
'about' is an insertion indicator.
'litre' becomes 'L' (abbreviation).
'tripe' enclosing 'l' is 'TRIPLE'.
'being' acts as a link.
(Other definitions for triple that I've seen before include "Increase threefold" , "Multiply by three" , "In three parts" , "Threefold, treble" , "make score sixty?" .)