The most common and most likely answer to this crossword clue is the 9 letter word NIGHTHAWK
I've seen this crossword clue in Independent
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More clues leading to the result NIGHTHAWK
Buddy explains!

The straight part
We know that "dark lover" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dark lover = nighthawk.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 NIGHTHAWK mean?
NIGHTHAWK noun- A nightjar, especially Caprimulgus europaeus.
- A New World nightjar of the genus Chordeiles, especially Chordeiles minor.
- A person whose preference or custom is to remain awake and active during the night and the early morning hours.
- A nighthawker.