Not popular: as one penning trendy verse? (9)
I believe the answer is:
uninvited
'not popular' is the definition.
'one penning trendy verse?' is the wordplay.
'one' becomes 'united' ('united' can be a synonym of 'one'**).
'penning' is an insertion indicator.
'trendy' becomes 'in'.
'verse?' becomes 'v'.
'in'+'v'='inv'
'united' placed around 'inv' is 'UNINVITED'.
'as' is the link.
(Other definitions for uninvited that I've seen before include "Gatecrasher" , "not wanted" , "Intruder" .)