I mention doctor very quickly (2,2,4)
I believe the answer is:
in no time
'very quickly' is the definition.
(I've seen this in another clue)
'i mention doctor' is the wordplay.
'doctor' indicates anagramming the letters.
'mention' is an anagram of 'nnotime'.
'i'+'nnotime'='IN NO TIME'
(Other definitions for in no time that I've seen before include "Very rapidly, hardly measurable" , "Very quickly" , "Swiftly" , "Very rapidly - but sounds impossible?" , "Without delay, too quick to measure" .)