Copper entering street, a tail (4)
I believe the answer is:
scut
'a tail' is the definition.
(I know that scut is a type of tail)
'copper entering street' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'entering' indicates putting letters inside.
'street' becomes 'st' (abbreviation).
'cu' put inside 'st' is 'SCUT'.
(Other definitions for scut that I've seen before include "Small tail as of rabbit" , "(Hare's) short upright tail" , "Short erect tail" , "Short tail of hare" , "Reindeer tail" .)