The most common and most likely answer to this crossword clue is the 7 letter word LUCERNE
I've seen this crossword clue in The Daily Telegraph
L
U
C
E
R
N
E
More clues leading to the result LUCERNE
- Eagle with endless pluck going over lake
- City relieving poor Uncle Remus of total raised
- Report of maybe escaped eagle in city
- Soldiers caught in retreat trapped by river somewhere in Switzerland
- Sea Eagle follows Little Cloud, initially over posh Swiss city
- The stars briefly meet bird in Swiss city
- Alfalfa‘s fortune curtailed by an eagle
- Alfalfa from Swiss lake
- Uncle swimming across English river and lake
- Forage crop in Swiss canton
Buddy explains!

The straight part
We know that "lake" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lake = lucerne.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 LUCERNE mean?
LUCERNE noun- Alfalfa.