Prey dipped into by rorqual's mouth, main diet? (5)
I believe the answer is:
krill
'main diet?' is the definition.
('the main' is a historic name for the sea)
'prey dipped into by rorqual's mouth' is the wordplay.
'prey' becomes 'kill' (I've seen this before).
'dipped into by' indicates putting letters inside.
'mouth' says to take the initial letters (mouth can mean something's entrance).
The initial letter of 'rorquals' is 'r'.
'kill' going around 'r' is 'KRILL'.
(Other definitions for krill that I've seen before include "Tiny shrimps" , "Food of whalebone whales" , "Tiny crustaceans" , "Whale food" , "Small shrimp-like creatures eaten by whales" .)