Back-to-front, entered bar (5)
I believe the answer is:
ingot
'bar' is the definition.
(block of metal, eg a bar of gold)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ingot that I've seen before include "Piece of cast metal" , "Gold Stick" , "Lump of gold etc" , "A lump of metal, perhaps gold" , "Pig" .)