Upset travellers come back carrying silverware (7)
I believe the answer is:
scupper
'upset' is the definition.
(scupper can mean to thwart or upset)
'travellers come back carrying silverware' is the wordplay.
'travellers' becomes 'reps' (as in sales reps).
'come back' is a reversal indicator.
'carrying' is an insertion indicator.
'silverware' becomes 'cup' (a cup might be an item of silverware).
'reps' written backwards gives 'sper'.
'sper' placed around 'cup' is 'SCUPPER'.
(Other definitions for scupper that I've seen before include "Ruin (plans)" , "Sink one's own ship deliberately" , "Deliberately sink (oneself)" , "foil" , "Cause to sink" .)