Remarkable person welcoming appearance as witness (8)
I believe the answer is:
onlooker
'witness' is the definition.
(I've seen this before)
'remarkable person welcoming appearance' is the wordplay.
'remarkable person' becomes 'oner' (slang for a unique person or thing).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'appearance' becomes 'look' (look is a kind of appearance).
'oner' placed around 'look' is 'ONLOOKER'.
'as' is the link.
(Other definitions for onlooker that I've seen before include "Spectator" , "One who watches without becoming involved" , "He stands and watches" , "One observes" , "Non-participating observer" .)