Witnesses are upon attractive people (9)
I believe the answer is:
onlookers
'witnesses' is the definition.
(I know this)
'upon attractive people' is the wordplay.
'upon' becomes 'on' (I've seen this in another clue).
'attractive people' becomes 'lookers' (I've seen this before).
'on'+'lookers'='ONLOOKERS'
'are' acts as a link.
(Other definitions for onlookers that I've seen before include "They watch" , "Bystanders, rubber-neckers" , "Spectators" .)