About to notice, for instance, returning birds (5)
I believe the answer is:
geese
'birds' is the definition.
(goose is a kind of bird)
'about to notice for instance returning' is the wordplay.
'about' indicates putting letters inside.
'to notice' becomes 'see' (notice can mean to spot or see).
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'returning' says the letters should be written backwards.
'see' going inside 'eg' is 'eseeg'.
'eseeg' in reverse letter order is 'GEESE'.
(Other definitions for geese that I've seen before include "Web-footed migratory aquatic birds" , "They come in gaggles and skeins" , "Long-necked, migratory aquatic birds" , "Skein or gaggle?" , "Large water birds" .)