Sift ideas about being happy (9)
I believe the answer is:
satisfied
'being happy' is the definition.
(thesaurus)
'sift ideas about' is the wordplay.
'about' indicates anagramming the letters.
'sift'+'ideas'='siftideas'
'siftideas' anagrammed gives 'SATISFIED'.
(Other definitions for satisfied that I've seen before include "Met" , "Content" , "Pleased, contented" , "Convinced" , "given answer > was convincing" .)