Sex position ordinary with Romeo's guest (7)
I believe the answer is:
visitor
'guest' is the definition.
(I've seen this before)
'sex position ordinary with romeo's' is the wordplay.
'sex' becomes 'VI' (VI in Roman numerals - 'sex' is 'six' in Latin).
'position' becomes 'sit' (sitting is a kind of positioning).
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'with' means one lot of letters go next to another.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'vi'+'sit'+'o'+'r'='VISITOR'
(Other definitions for visitor that I've seen before include "Company" , "Caller, guest" , "One coming to call" , "Someone calling (again at 23?)" , "Person turning up" .)