In the autumn, one cold could be infectious (6)
I believe the answer is:
septic
'be infectious' is the definition.
('septic' can be similar in meaning to 'infectious')
'in the autumn one cold' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'cold' could be 'c' (eg on taps) and 'c' is located in the answer.
The remaining letters 'sept' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'could' is the link.
Can you help me to learn more?
(Other definitions for septic that I've seen before include "Poisoned" , "Infected with bacteria" , "Contaminated with disease-causing bacteria" , "Type of tank" , "Caused by putrefaction" .)