U-boat is turned into a popular vehicle (3)
I believe the answer is:
bus
'a popular vehicle' is the definition.
'bus' can be an answer for 'vehicle' (bus is a kind of vehicle). I'm unsure of the 'a popular' bit.
'u-boat is turned' is the wordplay.
'u-boat' becomes 'sub' (German submarine).
'is turned' is a reversal indicator.
'sub' back-to-front is 'BUS'.
'into' acts as a link.
Can you help me to learn more?
(Other definitions for bus that I've seen before include "Something caught" , "eg Double-decker" , "Mode of public transport" , "Use transport" , "Means of public transport" .)