Motorists finally crossed waterways (7)
I believe the answer is:
drivers
'motorists' is the definition.
(I've seen this before)
'finally crossed waterways' is the wordplay.
'finally' says to take the final letters.
'waterways' becomes 'rivers' (I've seen this in another clue).
The final letter of 'crossed' is 'd'.
'd'+'rivers'='DRIVERS'
(Other definitions for drivers that I've seen before include "Golf clubs" , "What PC needs" , "9 [CLUBS]" , "People taking cars" , "Chauffeurs" .)