A third of biscuit, absorbed by beverage, is lost (2,3)
I believe the answer is:
at sea
'is lost' is the definition.
(both can mean confused or in trouble)
'a third of biscuit absorbed by beverage' is the wordplay.
'third of biscuit' becomes 's' (3rd letter of 'biscuit').
'absorbed by' is an insertion indicator.
'beverage' becomes 'tea' (tea is a kind of beverage).
's' inserted inside 'tea' is 'tsea'.
'a'+'tsea'='AT SEA'
(Other definitions for at sea that I've seen before include "Bewildered on the ocean?" , "stumped" , "Bewildered; sailing" , "of no fixed abode" , "Sailing off shore" .)