Splendid friend with a wagging tail (8)
I believe the answer is:
palatial
'splendid' is the definition.
(thesaurus)
'friend with a wagging tail' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'with' is an insertion indicator.
'wagging' indicates anagramming the letters (letters swing or wag around).
'tail' is an anagram of 'tial'.
'a' put within 'tial' is 'atial'.
'pal'+'atial'='PALATIAL'
(Other definitions for palatial that I've seen before include "Magnificently appointed" , "(Of building) grandly spacious" , "Spacious and splendid like royal home" , "Sumptuous and spacious" , "Hugely impressive" .)