Lecturer's about to make careless mistake (3)
I believe the answer is:
nod
'careless mistake' is the definition.
(I've seen this before)
'lecturer's about' is the wordplay.
'lecturer' becomes 'don' (I've seen this before).
'about' shows that the letters should be reversed in order.
'don' in reverse letter order is 'NOD'.
'to make' acts as a link.
(Other definitions for nod that I've seen before include "Appear to fall asleep" , "Agree non-verbally" , "slight bow" , "sleepy land" , "Drop the head" .)