Serious complaint originally brought up by our corporal in New Guinea (7,6)
I believe the answer is:
bubonic plague
'serious complaint' is the definition.
(serious disease)
'originally brought up by our corporal in new guinea' is the wordplay.
I cannot quite see how this works, but
'corporal' could be 'cpl' (abbreviation) and 'cpl' is present in the answer.
'new' could be 'n' (common abbreviation eg NT for New Testament) and 'n' is found in the answer.
This explanation may well be incorrect...
(Other definitions for bubonic plague that I've seen before include "Black Death" , "Disease transmitted by the rat flea's bite" .)