Threesome finally toast with port (4)
I believe the answer is:
trio
'threesome' is the definition.
(both can mean a group of three)
'finally toast with port' is the wordplay.
'finally' suggests the final letters.
'with' is a charade indicator (letters next to each other).
'port' becomes 'rio' (Rio de Janeiro is a port).
The last letter of 'toast' is 't'.
't'+'rio'='TRIO'
(Other definitions for trio that I've seen before include "Beethoven's 'Archduke': famous piece written for this ensemble" , "Riot (anag)" , "Small group playing" , "Beethoven's 'Archduke', for instance" , "Ensemble musical piece, eg Beethoven's 'Archduke'" .)