Chefs almost rule out being genial (8)
I believe the answer is:
cheerful
'genial' is the definition.
(I've seen this before)
'chefs almost rule out' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'chefs' with its last letter removed is 'chef'.
'chef'+'rule'='chefrule'
'chefrule' anagrammed gives 'CHEERFUL'.
'being' acts as a link.
(Other definitions for cheerful that I've seen before include "In a happy mood" , "Showing good spirits" , "Happy and optimistic" , "In good spirits, lighthearted" , "Contented" .)