The most common and most likely answer to this crossword clue is the 6 letter word CANNOT
I've seen this crossword clue in The Sunday Telegraph
C
A
N
N
O
T
Buddy explains!

The straight part
We know that "Lacks ability" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacks ability = cannot.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.
What does CANNOT mean?
CANNOT noun- Something that cannot be done.
- Can not (be unable to).
- Be forbidden or not permitted to