Almost every seabird fed every other (9)
I believe the answer is:
alternate
'every other' is the definition.
(synonyms)
'almost every seabird fed' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'every' becomes 'all' (similar in meaning).
'seabird' becomes 'tern' (tern is a kind of seabird).
'fed' becomes 'ate' ('eat' can be a synonym of 'feed').
'all' with its last letter taken away is 'al'.
'al'+'tern'+'ate'='ALTERNATE'
(Other definitions for alternate that I've seen before include "Act reciprocally" , "Every second one of a series" , "Substitute" , "Take turns" , "Change from one thing to another by turns" .)