The most common and most likely answer to this crossword clue is the 8 letter word LIFELESS
I've seen this crossword clue in The Guardian
L
I
F
E
L
E
S
S
More clues leading to the result LIFELESS
Buddy explains!

The straight part
We know that "Lacklustre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lacklustre = lifeless.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 LIFELESS mean?
LIFELESS adjective- Inanimate; having no life
- Dead; having lost life
- Uninhabited, or incapable of supporting life
- Dull or lacking vitality