Birds' price, not originally megabucks (9)
I believe the answer is:
ostriches
'birds'' is the definition.
(ostrich is a kind of bird)
'price not originally megabucks' is the wordplay.
'price' becomes 'cost' (synonyms).
'not originally' means to remove the first letter (the origin of the word is removed).
'megabucks' becomes 'riches' (I've seen this in another clue).
'cost' with its initial letter taken off is 'ost'.
'ost'+'riches'='OSTRICHES'
(Other definitions for ostriches that I've seen before include "Largest living birds" , "Fast-running flightless birds" , "People not facing the reality" .)