Amateur regularly left wrong score in the Open (8)
I believe the answer is:
alfresco
'in the open' is the definition.
(as in alfresco dining)
'amateur regularly left wrong score' is the wordplay.
'amateur' becomes 'a'.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'wrong' is an anagram indicator.
The alternating letters of 'left' are 'lf'.
'score' anagrammed gives 'resco'.
'a'+'lf'+'resco'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "Meal eaten in the outdoors" , "Outdoors, meal perhaps" , "Out of doors" , "Outside" , "out of the house" .)