Always wet behind the ears, but always popular (9)
I believe the answer is:
evergreen
'always popular' is the definition.
(I've seen this before)
'always wet behind the ears' is the wordplay.
'always' becomes 'ever' (synonyms).
'wet behind the ears' becomes 'green' (synonyms).
'ever'+'green'='EVERGREEN'
'but' acts as a link.
(Other definitions for evergreen that I've seen before include "Durable" , "having no garden shed?" , "Ageless" , "Continually fresh" , "Not dropping leaves" .)