Finished twentieth originally in the Open (5)
I believe the answer is:
overt
'in the open' is the definition.
(something overt is obvious or out in the open)
'finished twentieth originally' is the wordplay.
'finished' becomes 'over' (similar in meaning).
'originally' says to take the initial letters.
The initial letter of 'twentieth' is 't'.
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "Open, not 1 across" , "Plain and obvious" , "Voter (anag.)" , "Open, up front" , "Open and obvious" .)