Squawk by old bird, though not old bird! (5)
I believe the answer is:
macaw
'bird' is the definition.
(type of parrot)
'squawk by old bird though not old' is the wordplay.
'squawk' becomes 'caw' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' is a charade indicator (letters next to each other).
'old bird' becomes 'moa' (extinct bird).
'though not' suggests deleting specific letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'moa' with 'o' removed is 'ma'.
'caw' after 'ma' is 'MACAW'.
Can you help me to learn more?
(Other definitions for macaw that I've seen before include "Brightly-coloured parrot" , "Long-tailed American parrot" , "Largest parrot" , "New World parrot" , "Large bright parrot" .)