A doctor not one for a dull speech (8)
I believe the answer is:
monotone
'dull speech' is the definition.
'monotone' can be an answer for 'speech' (monotone is a kind of speech). I'm unsure of the 'dull' bit.
'a doctor not one for a' is the wordplay.
'a doctor' becomes 'mo' (medical officer).
'one for' is a charade indicator (letters next to each other).
'a' becomes 'one'.
'mo'+'not'+'one'='MONOTONE'
Can you help me to learn more?
(Other definitions for monotone that I've seen before include "Single unvaried sound level" , "Flat voice" , "Single unvarying note" , "Utterance at a single unvaried level" , "Dull delivery" .)