A certain atmosphere in the restaurant (4)
I believe the answer is:
aura
'a certain atmosphere' is the definition.
(aura can mean an atmosphere or mood)
'in the restaurant' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AURA' can be found hidden inside 'restaurant'.
(Other definitions for aura that I've seen before include "Mystic air" , "Air, distinctive quality" , "Nimbus" , "Characteristic air of person" , "Distinctive but intangible quality surrounding a person" .)