The old woman to have sex with Romeo (5)
I believe the answer is:
mater
'the old woman' is the definition.
(both can mean one's mother)
'sex with romeo' is the wordplay.
'sex' becomes 'mate' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'mate'+'r'='MATER'
'to have' acts as a link.
(Other definitions for mater that I've seen before include "See 12" , "having children" , "Mother (Latin or old slang)" , "One marrying" , "An old Roman mother" .)