Rubbish doctor initially seems so retrogressive (5)
I believe the answer is:
dross
'rubbish' is the definition.
(I've seen this before)
'doctor initially seems so retrogressive' is the wordplay.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor'**).
'initially' indicates taking the first letters.
'retrogressive' is a reversal indicator.
The initial letter of 'seems' is 's'.
's'+'so'='sso'
'sso' written backwards gives 'oss'.
'dr'+'oss'='DROSS'
(Other definitions for dross that I've seen before include "Lees" , "Rubbish; impurities" , "Something of inferior quality" , "Scum on molten metal" , "Worthless stuff" .)