Can it never quite crop up in adults? (5)
I believe the answer is:
croup
'can it never quite' is the definition.
I can't judge whether this defines the answer.
'crop up in adults?' is the wordplay.
'crop' means to remove the last letter.
'in' means one lot of letters goes inside another.
'adults?' becomes 'crop' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up' with its final letter taken away is 'u'.
'u' placed within 'crop' is 'CROUP'.
Can you help me to learn more?
(Other definitions for croup that I've seen before include "Illness" , "Hindquarters of a horse" , "Disease of the young with fever and breathing difficulty" , "Complaint of small children" , "Children's disease causing difficulty in breathing" .)