One watching Roman poet leaving right before (9)
I believe the answer is:
vigilante
'one watching' is the definition.
(I know this)
'roman poet leaving right before' is the wordplay.
'roman poet' becomes 'virgil'.
'leaving' is a deletion indicator.
'right' becomes 'r' (common abbreviation).
'before' becomes 'ante' (prefix meaning 'before').
'virgil' with 'r' taken out is 'vigil'.
'vigil'+'ante'='VIGILANTE'
(Other definitions for vigilante that I've seen before include "Self-appointed lawman" , "Man on patrol" , "One taking the law into his own hands" , "Unofficial policeman" , "Unauthorised law enforcer" .)