Back providing new seat for the party (6)
I believe the answer is:
fiesta
'the party' is the definition.
(I've seen this before)
'back providing new seat' is the wordplay.
'back' says the letters should be written in reverse.
'providing' becomes 'if' (I've seen this in other clues).
'new' is an anagram indicator.
'if' written backwards gives 'fi'.
'seat' with letters rearranged gives 'esta'.
'fi'+'esta'='FIESTA'
'for' is the link.
(Other definitions for fiesta that I've seen before include "A public holiday in Barcelona" , "Spanish party" , "religious celebration" , "(Iberian) religious holiday" , "(Spanish) religious festival" .)