Bar's very popular, understood? (5)
I believe the answer is:
ingot
'bar's' is the definition.
(block of metal, eg a bar of gold)
'very popular understood?' is the wordplay.
'very popular' becomes 'in' ('in' can mean trendy. I am not sure about the 'very' bit.).
'understood?' becomes 'got' (getting is a kind of understanding).
'in'+'got'='INGOT'
Can you help me to learn more?
(Other definitions for ingot that I've seen before include "Block of, eg, gold" , "Precious metal bar" , "A lump of metal like gold" , "Unwrought metal cast in a mould" , "Mass of metal, especially gold or silver" .)