Gloomy pioneer regularly associated with planet (9)
I believe the answer is:
saturnine
'gloomy' is the definition.
(thesaurus)
'pioneer regularly associated with planet' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'associated with' is a charade indicator (letters next to each other).
'planet' becomes 'saturn' (Saturn is an example**).
The alternating letters of 'pioneer' are 'ine'.
'ine' after 'saturn' is 'SATURNINE'.
(Other definitions for saturnine that I've seen before include "shining? The opposite" , "Turn in sea makes one gloomy" , "offering little amusement" , "Morose looking" , "Gloomy; dark and brooding" .)