Blair seems endlessly, terribly wretched (9)
I believe the answer is:
miserable
'wretched' is the definition.
('miserable' can be a synonym of 'wretched')
'blair seems endlessly terribly' is the wordplay.
'endlessly' means to remove the last letter.
'terribly' is an anagram indicator.
'blair'+'seems'='blairseems'
'blairseems' with its last letter taken away is 'blairseem'.
'blairseem' with letters rearranged gives 'MISERABLE'.
(Other definitions for miserable that I've seen before include "Wretched, doleful" , "Glum, wretched" , "Wretchedly unhappy" , "Dreary - pathetic" , "Depressed" .)