Atmosphere in fiesta I relished (3)
I believe the answer is:
air
'atmosphere' is the definition.
('air' can be a synonym of 'atmosphere')
'in fiesta i relished' is the wordplay.
'in' says the answer is hidden in the clue.
'AIR' can be found hidden inside 'fiesta i relished'.
(Other definitions for air that I've seen before include "It's a mixture mainly of oxygen and nitrogen" , "Look - warm and dry" , "Ventilate with oxygen" , "- brick; - bed; - brush" , "Light wind; song" .)