Motorist finally crossed waterway (6)
I believe the answer is:
driver
'motorist' is the definition.
(thesaurus)
'finally crossed waterway' is the wordplay.
'finally' indicates one should take the final letters.
'waterway' becomes 'river' (I've seen this before).
The final letter of 'crossed' is 'd'.
'd'+'river'='DRIVER'
(Other definitions for driver that I've seen before include "chauffeuse" , "Golf club; chauffeur" , "Member of RAC" , "Motorist - golf club" , "See 1 down" .)