Infected college by start of autumn? (6)
I believe the answer is:
septic
'infected' is the definition.
(I know that infected can be written as septic)
'college by start of autumn?' is the wordplay.
'college' becomes 'c'.
'by' means one lot of letters go next to another.
'start of autumn?' becomes 'Sept I' (resembles Sept 1, first of September).
'c' after 'septi' is 'SEPTIC'.
(Other definitions for septic that I've seen before include "Relating to putrefaction" , "Of wound, infected with pus" , "Of wound, infected and pus-filled" , "Poisonous" , "Infected with bacteria" .)