The competition allows both professional and amateur entrants (4)

I believe the answer is:

open

(Other definitions for open that I've seen before include "Unrestricted; ready for business" , "Candid and undisguised" , "Candid or unlocked" , "With which to offer hearty welcome" , "Candid - ready for business" .)

I've seen this clue in The Irish Times.

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