Dope mostly affected way of speaking (5)
I believe the answer is:
twerp
'dope' is the definition.
(I've seen this before)
'mostly affected way of speaking' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'affected' becomes 'twee' (I've seen this before).
'way of speaking' becomes 'RP' (Received Pronunciation, standard British accent).
'twee' with its final letter taken off is 'twe'.
'twe'+'rp'='TWERP'
(Other definitions for twerp that I've seen before include "Silly-billy" , "Nincompoop" , "Twit" , "Contemptible person" , "Ninny" .)