In new fashion, rule chef must be gay (8)
I believe the answer is:
cheerful
'gay' is the definition.
('cheerful' can be a synonym of 'gay')
'in new fashion rule chef' is the wordplay.
'in new fashion' indicates anagramming the letters (I've seen this in other clues).
'rule'+'chef'='rulechef'
'rulechef' anagrammed gives 'CHEERFUL'.
'must be' is the link.
(Other definitions for cheerful that I've seen before include "Happy and optimistic" , "Lure chef into being in good spirits" , "Contented" , "In good humour, quite jolly" , "Optimistic and upbeat" .)