U-boat coming about provides transport (3)
I believe the answer is:
bus
'transport' is the definition.
(busing is a kind of transporting)
'u-boat coming about' is the wordplay.
'u-boat' becomes 'sub' (German submarine).
'coming about' says the letters should be written backwards (about can mean backwards or around).
'sub' reversed gives 'BUS'.
'provides' acts as a link.
(Other definitions for bus that I've seen before include "Road vehicle able to carry many passengers" , "Public service vehicle" , "Public transport vehicle" , "Something caught" , "People carrier" .)