Copper led around small account, but it's a dead end (3,2,3)
I believe the answer is:
cul de sac
'it's a dead end' is the definition.
(I've seen this before)
'copper led around small account' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'around' indicates an anagram.
'small' becomes 's' (abbreviation - e.g. clothes size).
'account' becomes 'ac' (abbreviation for account).
'led' anagrammed gives 'lde'.
'cu'+'lde'+'s'+'ac'='CUL-DE-SAC'
'but' acts as a link.
(Other definitions for cul de sac that I've seen before include "Dead-end road" , "Blind alley" , "Block end; no through road" , "Dead end street" .)