Zero cost for lecture (5)
I believe the answer is:
orate
'lecture' is the definition.
(I've seen this before)
'zero cost' is the wordplay.
'zero' becomes 'o'.
'cost' becomes 'rate' (rate is a kind of cost).
'o'+'rate'='ORATE'
'for' is the link.
(Other definitions for orate that I've seen before include "Talk pompously" , "Speaker may" , "Spout" , "Make a flowery speech" , "Erato (anag.)" .)