It facilitates travel for a girl with sex appeal (4)
I believe the answer is:
visa
'it facilitates travel' is the definition.
The definition suggests a singular noun which matches the answer.
'girl with sex appeal' is the wordplay.
'girl' becomes 'vi' (short for Violet).
'with' says to put letters next to each other.
'sex appeal' becomes 'SA' (old-fashioned abbreviation).
'vi'+'sa'='VISA'
'for a' is the link.
Can you help me to learn more?
(Other definitions for visa that I've seen before include "Travel stamp in passport" , "Travel document is required to enter Virginia" , "Authorisation to enter a country" , "It allows the bearer to enter the country issuing it" , "Endorsement made in a passport" .)