Possible Clues:

• Thick of things
• Sidestroke, e.g.
• Soon with impetus make, initially, progress in water
• Take more than one stroke
• Emulate Michael Phelps
• At . . . . Two Birds by Flann O'Brien
• Make progress by crawling, perhaps
• Triathlon leg
• 1960s dance, with 'the'
• Crawl from the Devon and Cornwall motorway northwards