Luxurious pad mostly, one in a tall building (8)
I believe the answer is:
palatial
'luxurious' is the definition.
(I've seen this before)
'pad mostly one in a tall building' is the wordplay.
'mostly' means to remove the last letter.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'building' indicates an anagram.
'pad' with its final letter taken off is 'pa'.
'a'+'tall'='atall'
'atall' is an anagram of 'latal'.
'i' put within 'latal' is 'latial'.
'pa'+'latial'='PALATIAL'
(Other definitions for palatial that I've seen before include "Huge, luxurious (rooms)" , "Luxurious - magnificent" , "Of building, spacious and splendid, fit for a king" , "Big, luxurious (accommodation)" , "Spacious and splendid like royal home" .)