Being unwell, I had to conceal terrible dopiness (10)
I believe the answer is:
indisposed
'being unwell' is the definition.
('indisposed' can be a synonym of 'unwell')
'i had to conceal terrible dopiness' is the wordplay.
'i had' becomes 'id' (I'd).
'to conceal' means one lot of letters goes inside another.
'terrible' indicates anagramming the letters.
'dopiness' with letters rearranged gives 'ndispose'.
'id' going around 'ndispose' is 'INDISPOSED'.
(Other definitions for indisposed that I've seen before include "Out of sorts" , "Slightly unwell" , "Poorly" .)