Retired lecturer gives green light (3)
I believe the answer is:
nod
'green light' is the definition.
(I've seen this before)
'retired lecturer' is the wordplay.
'retired' says the letters should be written in reverse.
'lecturer' becomes 'don' (I've seen this before).
'don' in reverse letter order is 'NOD'.
'gives' acts as a link.
(Other definitions for nod that I've seen before include "Indicate assent; make slip" , "Gesture of agreement" , "Move head in agreement" , "Gesture; agree" , "Show recognition" .)