Jam finally reaching clear section of waterway (8)
I believe the answer is:
gridlock
'jam' is the definition.
(I've seen this before)
'finally reaching clear section of waterway' is the wordplay.
'finally' suggests the final letters.
'clear' becomes 'rid' ('rid' can be a synonym of 'clear').
'section of waterway' becomes 'lock'.
The last letter of 'reaching' is 'g'.
'g'+'rid'+'lock'='GRIDLOCK'
(Other definitions for gridlock that I've seen before include "Standstill" , "Traffic snarl-up" , "Total traffic jam" , "Severe traffic jam" , "Traffic jam at intersections" .)