Register to collect old bird (7)
I believe the answer is:
rooster
'bird' is the definition.
(rooster is a kind of bird)
'register to collect old' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
'register' could be 'roster' (I've seen this in another clue) and 'roster' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rooster that I've seen before include "Morning caller" , "Early-morning caller" , "Adult male domestic fowl" , "Farmyard cock" , "One may crow" .)