Discuss when the lecture's finished? (4,4)
I believe the answer is:
talk over
'discuss' is the definition.
(I know that discuss can be written as talk over)
'the lecture's finished' is the wordplay.
'the lecture' becomes 'talk' ('talk' can be a synonym of 'lecture').
'finished' becomes 'over' (similar in meaning).
'talk'+'over'='TALK OVER'
'when' is the link.
(Other definitions for talk over that I've seen before include "Discuss" , "Consider together - persuade" .)