Best starts to opening round in The Open? (7)
I believe the answer is:
outdoor
'in the open?' is the definition.
(I've seen this before)
'best starts to opening round' is the wordplay.
'best' becomes 'outdo' (synonyms).
'starts to' says to take the initial letters.
The initial letters of 'opening round' is 'or'.
'outdo'+'or'='OUTDOOR'
(Other definitions for outdoor that I've seen before include "Alfresco - sort of pursuits" , "exposed to the elements" , "Done in the open" , "Open-air" , "healthy pursuits" .)