Illness lad incubated in a period of days (6)
I believe the answer is:
malady
'illness' is the definition.
(synonyms)
'lad incubated in a period of days' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'period' could be 'ad' (AD is an example) and 'ad' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for malady that I've seen before include "Sickness, ailment" , "Ailment; illness" , "Disease, ailment" , "affection" , "hay fever, perhaps" .)