Insignificant litre in whisky, I'm opening case of sake! (5-4)
I believe the answer is:
small time
'insignificant' is the definition.
(I've seen this before)
'litre in whisky i'm opening case of sake' is the wordplay.
'litre' becomes 'L' (abbreviation).
'in' is an insertion indicator.
'whisky' becomes 'malt' (malt whisky is a kind of whisky).
'opening' indicates putting letters inside (some letters open a gap in others).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'sake' with its centre removed is 'se'.
'l' going within 'malt' is 'mallt'.
'mallt'+'im'='malltim'
'malltim' inserted inside 'se' is 'SMALL-TIME'.
(Other definitions for small time that I've seen before include "Mo may be so" , "Petty (criminal, usually)" , "Insignificant" , "Unimportant, minor" , "Minor (colloq.)" .)