Original opening I have, popular at first (9)
I believe the answer is:
inventive
'original' is the definition.
(I've seen this before)
'opening i have popular at first' is the wordplay.
'opening' becomes 'vent' (vent is a kind of opening**).
'i have' becomes 'ive'.
'popular' becomes 'in'.
'at first' says to put letters next to each other.
'vent'+'ive'='ventive'
'ventive' after 'in' is 'INVENTIVE'.
(Other definitions for inventive that I've seen before include "original" , "showing ingenuity" , "Creative" .)