Erred carelessly, taking on a senior lecturer (6)
I believe the answer is:
reader
'senior lecturer' is the definition.
(I've seen this before)
'erred carelessly taking on a' is the wordplay.
'carelessly' indicates anagramming the letters.
'taking on' indicates putting letters inside.
'erred' is an anagram of 'reder'.
'reder' going around 'a' is 'READER'.
(Other definitions for reader that I've seen before include "She lectures" , "University lecturer; book of educational extracts" , "Interpreter of what is written" , "Senior lecturer" , "(User of) standard textbook" .)