U-boat coming up provides transport (3)
I believe the answer is:
bus
'transport' is the definition.
(busing is a kind of transporting)
'u-boat coming up' is the wordplay.
'u-boat' becomes 'sub' (German submarine).
'coming up' shows that the letters should be reversed in order (in a down clue, letters go up).
'sub' in reverse letter order is 'BUS'.
'provides' acts as a link.
(Other definitions for bus that I've seen before include "eg Double-decker" , "Charabanc" , "Large passenger vehicle - small business!" , "Road vehicle able to carry many passengers" , "Public transport vehicle" .)