Manners shown by Dr. Dre? Not at first, in today's theme ... (5)
I believe the answer is:
mores
'manners' is the definition.
(I've seen this in another clue)
'dr dre? not at first in today's theme' is the wordplay.
'dr' becomes 'mo' (abbreviation for medical officer).
'not at first' means to remove the first letter.
'in today's theme' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'dre' with its first letter taken away is 're'.
'mo'+'re'+'s'='MORES'
'shown by' acts as a link.
(Other definitions for mores that I've seen before include "O tempora, O - !" , "Social customs of a group" , "Customs and conventions of society" , "Local customs and conventions" , "(Society's) conventions" .)