Few getting into road that's undulating (4)
I believe the answer is:
wavy
'undulating' is the definition.
(thesaurus)
'few getting into road' is the wordplay.
'few' becomes 'v'.
'getting into' indicates putting letters inside.
'road' becomes 'way' (I've seen this in other clues**).
'v' put into 'way' is 'WAVY'.
'that's' acts as a link.
(Other definitions for wavy that I've seen before include "Rippled" , "Curly (hair)" , "Undulating" , "Sinuous" , "full of bends" .)