As if diets could be happy (9)
I believe the answer is:
satisfied
'happy' is the definition.
(thesaurus)
'as if diets could be' is the wordplay.
'could be' indicates an anagram.
'as'+'if'+'diets'='asifdiets'
'asifdiets' with letters rearranged gives 'SATISFIED'.
(Other definitions for satisfied that I've seen before include "Pleased, contented" , "as one may be with a second helping" , "given answer > was convincing" , "served" , "Convinced" .)