Bird is injected with thiamine (4)
I believe the answer is:
ibis
'thiamine' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?
'bird is injected' is the wordplay.
'bird' becomes 'bi' (I can't justify this - if you can you should give a lot more credence to this answer).
'injected' indicates putting letters inside.
'bi' placed within 'is' is 'IBIS'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for ibis that I've seen before include "Large wading bird" , "leggy bird" , "Bird sacred in Ancient Egypt" , "One has large bill" , "Bird, sacred to ancient Egyptians" .)