Open's finished -- amateur's fourth (5)
I believe the answer is:
overt
'open's' is the definition.
(associated in meaning)
'finished amateur's fourth' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'amateur's fourth' becomes 't' (4th letter of 'amateur').
'over'+'t'='OVERT'
(Other definitions for overt that I've seen before include "____ is able to be seen" , "Trove (anag.)" , "Open and unconcealed" , "Open and obvious" , "Not locked away" .)