It could be fairest to exclude Republican Party (6)
I believe the answer is:
fiesta
'party' is the definition.
(I know that fiesta is a type of party)
'it could be fairest to exclude republican' is the wordplay.
'it could be' is an anagram indicator (the same letters could be an anagram).
'to exclude' indicates named letters should be taken away.
'republican' becomes 'r' (abbreviation).
'fairest' with 'r' removed is 'faiest'.
'faiest' anagrammed gives 'FIESTA'.
(Other definitions for fiesta that I've seen before include "Public celebration in Spain" , "religious celebration" , "Saint's day; carnival" , "A Spanish public celebration" , "Elaborate public party, in Spain perhaps" .)