Whale food makes a couple of kings sick (5)
I believe the answer is:
krill
'whale food' is the definition.
(I've seen this before)
'makes a couple of kings sick' is the wordplay.
'a couple of kings' means two replacements for 'king'.
'makes' indicates anagramming the letters.
'king' becomes 'r' (r is abbreviation for rex).
'king' becomes 'k'.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'r'+'k'='rk'
'rk' with letters rearranged gives 'kr'.
'kr'+'ill'='KRILL'
(Other definitions for krill that I've seen before include "Whale food" , "Small shrimp-like creatures eaten by whales" , "What baleen whales eat" , "seafood" , "Crustaceans, diet of whales" .)