Conclude drink menu's lacking space inside (3,2)
I believe the answer is:
sum up
'conclude' is the definition.
(I've seen this before)
'drink menu's lacking space inside' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'lacking' indicates named letters should be taken away.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'inside' is an insertion indicator.
'menu' with 'en' removed is 'mu'.
'sup' placed around 'mu' is 'SUM UP'.
(Other definitions for sum up that I've seen before include "Of a judge, give the main points of the evidence for jury" , "Conclude" , "Money more than before" , "(Of a judge) review the evidence" , "Recapitulate (in court)" .)