Doctor caught in error is a simple man (5)
I believe the answer is:
simon
'simple man' is the definition.
'simon' can be an answer for 'man' (I've seen this before). I'm not certain of the 'simple' bit.
'doctor caught in error' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'caught in' indicates putting letters inside.
'error' becomes 'sin' (a sin is a moral error).
'mo' going inside 'sin' is 'SIMON'.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for simon that I've seen before include "Apostle Peter's other name" , "- and Garfunkel" , "Bolivar, de Montfort, Callow or Cowell?" , "Simple pieman" , "Fellow" .)