Deal with motorway getting flooded by river (6)
I believe the answer is:
timber
'deal' is the definition.
(I've seen this before)
'motorway getting flooded by river' is the wordplay.
'motorway' becomes 'M' (as in motorway names - Mt, Me etc).
'getting flooded by' is an insertion indicator (some letters flood or overwhelm others).
'river' becomes 'tiber' (River Tiber in Italy).
'm' inserted inside 'tiber' is 'TIMBER'.
'with' acts as a link.
(Other definitions for timber that I've seen before include "Feller's warning cry" , "board" , "wooden" , "Watch out for falling tree!" , "Wood that's been cut and prepared" .)