Watch girl leaving right after sex (5)
I believe the answer is:
vigil
'watch' is the definition.
(synonyms)
'girl leaving right after sex' is the wordplay.
'leaving' is a deletion indicator.
'right' becomes 'R' (common abbreviation).
'after' says to put letters next to each other.
'sex' becomes 'VI' ('sex' is Latin for 'six' or VI in Roman numerals).
'girl' with 'r' taken away is 'gil'.
'gil' after 'vi' is 'VIGIL'.
(Other definitions for vigil that I've seen before include "A period of staying awake as for prayer" , "Watch maintained at night" , "Through the night, one kept" , "Period of 9" , "Night-time watch" .)