Rob sadly has to wait for witness (8)
I believe the answer is:
observer
'witness' is the definition.
(I've seen this before)
'rob sadly has to wait' is the wordplay.
'sadly' is an anagram indicator.
'has' means one lot of letters goes inside another.
'to wait' becomes 'serve' (to wait on someone is to serve them).
'rob' anagrammed gives 'obr'.
'obr' enclosing 'serve' is 'OBSERVER'.
'for' is the link.
(Other definitions for observer that I've seen before include "Someone watching" , "Non-participant; a paper" , "Viewer" , "Newspaper - witness" , "One watching attentively" .)