Way in by means of long avenue (5)
I believe the answer is:
vista
'long avenue' is the definition.
(vista can mean a long or narrow path with a view)
'way in by means of' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'in' means one lot of letters goes inside another.
'by means of' becomes 'via' (via can mean 'by means of' or 'using').
'st' put inside 'via' is 'VISTA'.
(Other definitions for vista that I've seen before include "Distant view" , "Broad prospect" , "Panorama" , "Panoramic view" , "Prospect, view" .)