Modern day teacher's computer accessory (3-2)
I believe the answer is:
add-on
'computer accessory' is the definition.
(add-on device for a computer)
'modern day teacher's' is the wordplay.
'modern day' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'teacher' becomes 'don' (don is a kind of teacher).
'ad'+'don'='ADD-ON'
(Other definitions for add-on that I've seen before include "Feature supplementing standard model" , "Extra bit" , "Computer optional extra" , "Computer peripheral" , "Feature bolted on to a standard model or package" .)