Lecturer's visible prompt (6)
I believe the answer is:
incite
'prompt' is the definition.
(I know that actuate can be written as incite)
'lecturer's visible' is the wordplay.
'lecturer's' indicates a 'sounds like' (homophone) clue (how it might sound in a lecture).
'visible' becomes 'insight' (something in sight is visible).
'insight' sounds like 'INCITE'.
(Other definitions for incite that I've seen before include "Stir up, urge on to action" , "Stir up (trouble)" , "Whip up" , "Stir up, rouse to action" , "Urge, instigate" .)