Clue: Prey dipped into by rorqual's mouth, main diet? (5)
Answer: KRILL
Explanation: Definition: main diet?
Definition: Krill are shrimplike creatures in the "main" i.e. the sea, eaten by whales, and rorquals are a type of whale
 **KILL**="Prey" with the opening/"mouth" of **R**[orqual] dipping inside