Cross-country’s such sport: better when it ends in gold! (7)
I believe the answer is:
outdoor
'cross-country's such sport' is the definition.
(I know this)
'better when it ends in gold' is the wordplay.
'better' becomes 'outdo' (to outdo is to beat or do better than).
'when it ends in' means one lot of letters go next to another (some letters go at the end of others).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'outdo'+'or'='OUTDOOR'
(Other definitions for outdoor that I've seen before include "in the garden, say" , "exposed to the elements" , "Done in the open" , "Not done in the house" , "Alfresco - sort of pursuits" .)