Bug a ship at sea (5)
I believe the answer is:
aphis
'bug' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'a ship at sea' is the wordplay.
'at sea' is an anagram indicator.
'ship' anagrammed gives 'phis'.
'a'+'phis'='APHIS'
Can you help me to learn more?
(Other definitions for aphis that I've seen before include "Greenfly" , "Small insect" , "sucker?" , "Plant louse" , "Sap-sucking garden pest" .)