Clean motorway entrance (5)
I believe the answer is:
charm
'entrance' is the definition.
(synonyms)
'clean motorway' is the wordplay.
'clean' becomes 'char' (char can mean a cleaner).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'char'+'m'='CHARM'
(Other definitions for charm that I've seen before include "Allure" , "Win over" , "Bracelet ornament" , "Greatly delight" , "Enrapture" .)