Traditional copper on street leading old girl (9)
I believe the answer is:
customary
'traditional' is the definition.
(thesaurus)
'copper on street leading old girl' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'on street' becomes 'st' (I've seen this in other clues).
'leading' says to take the initial letters.
'girl' becomes 'mary' (**).
The initial letter of 'old' is 'o'.
'cu'+'st'+'o'+'mary'='CUSTOMARY'
(Other definitions for customary that I've seen before include "In accordance with convention" , "Courts may find this usual" , "Conventional" , "Habitual, usual" , "precedent" .)