The most common and most likely answer to this crossword clue is the 13 letter word PROSOPAGNOSIA
I've seen this crossword clue in The Guardian
P
R
O
S
O
P
A
G
N
O
S
I
A
Buddy explains!
The straight part
We know that "Lacking 19 1/ across" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacking 19 1/ across = prosopagnosia.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
PROSOPAGNOSIA noun- A form of visual agnosia characterised by difficulty with face recognition despite intact low-level visual processing.