A lecturer welcomes 500 extra
I believe the answer is:
addon
'extra' is the definition.
(I've seen this before)
'a lecturer welcomes 500' is the wordplay.
'lecturer' becomes 'don' (don can mean an academic).
'welcomes' means one lot of letters goes inside another (in sense of 'invites in').
'500' becomes 'D' (Roman numeral for 500).
'don' going around 'd' is 'ddon'.
'a'+'ddon'='ADD-ON'
(Other definitions for addon that I've seen before include "Extra charge" , "something extra" .)