Always naive, having enduring popularity (9)
I believe the answer is:
evergreen
'having enduring popularity' is the definition.
(I've seen this before)
'always naive' is the wordplay.
'always' becomes 'ever' (synonyms).
'naive' becomes 'green' (similar in meaning).
'ever'+'green'='EVERGREEN'
(Other definitions for evergreen that I've seen before include "Holly, for example" , "Having an enduring freshness" , "Having enduring popularity" , "Hardy, no doubt" , "Plant with leaves all year round" .)