Plankton: prey rorquals principally swallowed (5)
I believe the answer is:
krill
'plankton' is the definition.
(type of plankton)
'prey rorquals principally swallowed' is the wordplay.
'prey' becomes 'kill' (I've seen this before).
'principally' suggests taking the first letters.
'swallowed' means one lot of letters goes inside another.
The first letter of 'rorquals' is 'r'.
'kill' enclosing 'r' is 'KRILL'.
(Other definitions for krill that I've seen before include "Crustaceans, diet of whales" , "Very small crustaceans" , "Whale food" , "Tiny crustaceans" , "Food of whalebone whales" .)