Possible Clues:

• Toward the back, as a fin
• Shark's back fin, e.g.
• Fin on the back of marine vertebrates
• Like some fins
• 'On the back, a fin on a fish perhaps (6)'
• Relating to the back
• Fishing pole raised by sailor nearly seeing fin
• ___ fin (shark feature)
• In The Zoo, part of the back door is missing second lock and the odd stall?
• Back fish fin