Uncontrollable fear overwhelms half of the medics who are upset by widespread disease (8)
I believe the answer is:
pandemic
'widespread disease' is the definition.
(I've seen this before)
'uncontrollable fear overwhelms half of the medics who are upset' is the wordplay.
'uncontrollable fear' becomes 'panic' (I've seen this before).
'overwhelms' means one lot of letters goes inside another.
'half of the medics who' becomes 'med' (I can't justify this - if you can you should give a lot more credence to this answer).
'are upset' says the letters should be written backwards.
'med' back-to-front is 'dem'.
'panic' going around 'dem' is 'PANDEMIC'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for pandemic that I've seen before include "Occurring over a wide area" , "extensive disease" , "Existing everywhere" , "(Of a disease) affecting a wide area" , "Widely spread disease" .)