Liqueur receipts mistakenly left inside (6,3)
I believe the answer is:
triple sec
'liqueur' is the definition.
(triple sec is a kind of liqueur)
'receipts mistakenly left inside' is the wordplay.
'mistakenly' is an anagram indicator.
'left' becomes 'l' (common abbreviation).
'inside' means one lot of letters goes inside another.
'receipts' is an anagram of 'tripesec'.
'tripesec' placed around 'l' is 'TRIPLE SEC'.
(Other definitions for triple sec that I've seen before include "alcohol" , "Drink" , "Strong orange-flavoured liqueur - perils etc (anag)" .)