Piece of bullion, for example, brought back in boat (5)
I believe the answer is:
barge
'boat' is the definition.
(I know that barge is a type of boat)
'piece of bullion for example brought back' is the wordplay.
'piece of bullion' becomes 'bar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for example' becomes 'e.g.' (short for 'exempli gratia', 'for example' in Latin).
'brought back' shows that the letters should be reversed in order.
'eg' back-to-front is 'ge'.
'bar'+'ge'='BARGE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for barge that I've seen before include "Canal boat, towed along (5)" , "Freight vessel" , "Violently push" , "Canal boat maybe" , "Shove, lighter" .)