Turning, lecturer is to indicate agreement (3)
I believe the answer is:
nod
'to indicate agreement' is the definition.
(I've seen this before)
'turning lecturer' is the wordplay.
'turning' says the letters should be written backwards.
'lecturer' becomes 'don' (I've seen this before).
'don' backwards is 'NOD'.
'is' is the link.
(Other definitions for nod that I've seen before include "What Homer may sometime do" , "OK" , "sleepy land" , "positive sign" , "sign of approval" .)