Complex sentence: On 17 March 1809, "Foxhound" was one of a fleet of 60 vessels of various kinds, under Admiral Lord Gambier, that anchored off Basque Roads to attack the French fleet lying within.
Split into simpler sentences: On 17 March 1809, "Foxhound" joined Admiral Lord Gambier's fleet anchored off the Basque Roads. Gambier's plan was to use his 60 vessels (of all kinds) to attack the French fleet lying within.