Popular entry, but not in the Open (6)
I believe the answer is:
indoor
'not in the open' is the definition.
(I've seen this before)
'popular entry' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'entry' becomes 'door' (synonyms).
'in'+'door'='INDOOR'
'but' is the link.
(Other definitions for indoor that I've seen before include "Extremely" , "not alfresco" , "Situated within a building - maybe a sport" , "Situated under cover" , "Describing sports such as squash" .)