Just a gathering of buyers and sellers (4)
I believe the answer is:
fair
'just' is the definition.
(just can mean fair in the sense of showing justice)
'a gathering of buyers and sellers' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
The remaining letters 'fir' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for fair that I've seen before include "Sign following Pisces" , "'Show, exhibition, market (4)'" , "Decent" , "Equitable ? - trade show" , "[OPPOSITE OF] Dark" .)