Bird — a crow, say — possibly eating spiders at first (9)
I believe the answer is:
cassowary
'bird' is the definition.
(cassowary is a kind of bird)
'a crow say possibly eating spiders at first' is the wordplay.
'possibly' indicates anagramming the letters.
'eating' is an insertion indicator.
'at first' suggests taking the first letters.
The first letter of 'spiders' is 's'.
'a'+'crow'+'say'='acrowsay'
'acrowsay' with letters rearranged gives 'casowary'.
'casowary' placed around 's' is 'CASSOWARY'.
(Other definitions for cassowary that I've seen before include "Large flightless bird - Oscar's way (anag)" , "Flightless bird related to the emu - Oscar's way (anag)" , "Winger" , "Large black flightless bird of N Australia" , "So a cry was changed of the running bird" .)