Short cut that's taken by copper (5)
I believe the answer is:
'copper' is the definition.
(I know that copper can be written as bobby)
'short cut that's taken by' is the wordplay.
'short cut' becomes 'bob' (I have seen 'Short cut' mean 'bob' so perhaps 'cut' could also mean 'bob'; 'short' suggests a short form).
'that's taken' means one lot of letters go next to another.
(Other definitions for bobby that I've seen before include "Policeman (sl.)" , "Policeman (colloq.)" , "Peeler" , "- Moore, World Cup winning captain" , "Policeman; man's name" .)