As I fed, it's becoming full (9)
I believe the answer is:
satisfied
'full' is the definition.
(thesaurus)
'as i fed it's becoming' is the wordplay.
'becoming' is an anagram indicator.
'as'+'i'+'fed'+'its'='asifedits'
'asifedits' with letters rearranged gives 'SATISFIED'.
(Other definitions for satisfied that I've seen before include "as one may be with a second helping" , "Content" , "Met" , "Complied with (a condition)" , "Well catered for" .)