Extra charge for university lecturer after a day (3-2)
I believe the answer is:
add-on
'extra charge' is the definition.
(I've seen this before)
'university lecturer after a day' is the subsidiary indication.
'university lecturer' becomes 'don' (I've seen this before).
'after' says to put letters next to each other.
'a day' becomes 'ad' (I've seen this in other clues).
'don' after 'ad' is 'addon'.
'for' is the link.
(Other definitions for add-on that I've seen before include "Extra charge" , "Feature supplementing standard model" , "Computer optional extra" , "Optional item as supplement or increment" , "Extra bit" .)