Splendid friend, regularly a stoic ally (8)
I believe the answer is:
palatial
'splendid' is the definition.
(thesaurus)
'friend regularly a stoic ally' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend**).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'a'+'stoic'+'ally'='astoically'
The alternate letters of 'astoically' are 'atial'.
'pal'+'atial'='PALATIAL'
(Other definitions for palatial that I've seen before include "Grandiose" , "Sumptuous, like a royal dwelling" , "Luxurious - magnificent" , "(Of building) grandly spacious" , "Huge, luxurious (rooms)" .)