German boy mounted popular British coin some years ago (6)
I believe the answer is:
florin
'british coin some years ago' is the definition.
(historical British coin)
'german boy mounted popular' is the wordplay.
'german boy' becomes 'rolf' (common German name).
'mounted' is a reversal indicator.
'popular' becomes 'in' ('in' can mean trendy).
'rolf' in reverse letter order is 'flor'.
'flor'+'in'='FLORIN'
(Other definitions for florin that I've seen before include "[SHILLINGS] ?" , "Old English coin worth 10p" , "Two bob, in old money" , "Old two-bob bit" , "Former British silver coin" .)