A blue one is hardly great for the ornithologist (3)
I believe the answer is:
tit
'a blue' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe there's a link between them I don't understand?
'one is hardly great for the ornithologist' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tit that I've seen before include "Boob made backing" , "Small garden bird" , "What you swap for tat" , "Small songbird" , "18's [BIRD]" .)