The best man, joined by Agnes for drinks (5)
I believe the answer is:
champ
'agnes for drinks' is the definition.
I can't tell whether this definition defines the answer.
'the best man joined' is the wordplay.
'the best' becomes 'cham' (I can't explain this - if you can you should give a lot more credence to this answer).
'man' becomes 'p' (abbreviation for pawn, a chess piece or 'man').
'joined' means one lot of letters go next to another.
'cham'+'p'='CHAMP'
'by' is the link.
Can you help me to learn more?
(Other definitions for champ that I've seen before include "boxer who is top dog?" , "Mashed potato with scallions" , "use teeth?" , "Fret impatiently; winner" , "Show impatience" .)