Do outside pub: be prompt (8)
I believe the answer is:
barbecue
'do outside' is the definition.
(type of outside event or do)
'pub be prompt' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'prompt' becomes 'cue' (cue can mean a prompt or signal).
'bar'+'be'+'cue'='BARBECUE'
(Other definitions for barbecue that I've seen before include "Alfresco grill" , "Alfresco meal" , "Cook on an open fire" , "A picnic with grilled meats" , "Cuba beer (anag.)" .)