The FELL crossword solution is referring to a type of mountain in the Lake District. A FELL is a high and barren landscape feature, such as a hill or mountain, found in northern England and Scotland. The Lake District is a mountainous region in North West England, containing some of the highest mountains in England, including Scafell Pike, which is the highest. Therefore, the phrase 'Somewhere Up in the Lake District' could be referring to any of the fells in the area.