Party is boring; stuffiest atmosphere (6)
I believe the answer is:
fiesta
'party' is the definition.
(I've seen this before)
'boring stuffiest atmosphere' is the wordplay.
'boring' indicates a hidden word (the solution has bored or cut into the clue).
'FIESTA' can be found hidden inside 'stuffiest atmosphere'.
'is' is the link.
(Other definitions for fiesta that I've seen before include "Continental knees-up" , "Festival, fete" , "foreign holiday" , "public event abroad" , "Elaborate public party, in Spain perhaps" .)