Bird and endlessly enormous fellow (5)
I believe the answer is:
titan
'enormous fellow' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'bird and endlessly' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'endlessly' means to remove the last letter.
'and' with its last letter removed is 'an'.
'tit'+'an'='TITAN'
Can you help me to learn more?
(Other definitions for titan that I've seen before include "Atlas, possibly" , "very big" , "Taint (anag.)" , "Jupiter's largest satellite" , "Colossus" .)