Cross after minor operation brought about complaint (8)
I believe the answer is:
smallpox
'complaint' is the definition.
(smallpox is a kind of health problem)
'cross after minor operation brought' is the wordplay.
I cannot really see how this works, but
'cross' could be 'x' (X is cross-shaped) and 'x' is found within the answer.
'minor' could be 'small' (similar in meaning) and 'small' is found in the answer.
The remaining letters 'po' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'about' is the link.
Can you help me to learn more?
(Other definitions for smallpox that I've seen before include "Viral disease, now almost eradicated by vaccination" , "Acute viral disease, technical name variola" , "Highly contagious viral disease with fever and skin pustules that leave permanent scars" .)