Nine hens have vet unexpectedly overjoyed (2,7,6)
I believe the answer is:
in seventh heaven
'overjoyed' is the definition.
(thesaurus)
'nine hens have vet unexpectedly' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'nine'+'hens'+'have'+'vet'='ninehenshavevet'
'ninehenshavevet' with letters rearranged gives 'IN SEVENTH HEAVEN'.
(Other definitions for in seventh heaven that I've seen before include "Rapturous" , "Jubilant" .)