Source of bubonic and different plague? (6)
I believe the answer is:
bother
'plague?' is the definition.
(plaguing is a kind of bothering)
'source of bubonic and different' is the wordplay.
'source of' suggests taking the first letters.
'and' says to put letters next to each other.
'different' becomes 'other' (similar in meaning).
The initial letter of 'bubonic' is 'b'.
'b'+'other'='BOTHER'
(Other definitions for bother that I've seen before include "Fuss; oh no!" , "Irritate - concern oneself" , "Annoy, disturb" , "Discommode, annoy" , "Annoy or pester" .)