In the open air better? Endlessly worse! (8)
I believe the answer is:
'in the open air' is the definition.
(I've seen this before)
'better? endlessly worse' is the wordplay.
'better?' becomes 'outdo' (bettering is a kind of outdoing**).
'endlessly' indicates the central letters (the ends of the words are removed).
The central letters of 'worse' are 'ors'.
(Other definitions for outdoors that I've seen before include "Not in a building" , "Al fresco" , "abroad" , "Is that great?" , "Alfresco" .)