Sickness worker Henry mentioned pains (7)
I believe the answer is:
anthrax
'sickness' is the definition.
(I know that anthrax is a type of disease)
'worker henry mentioned pains' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'mentioned pains' becomes 'rax' (I can't justify this - if you can you should give a lot more credence to this answer).
'ant'+'h'+'rax'='ANTHRAX'
Can you help me to learn more?
(Other definitions for anthrax that I've seen before include "Nasty illness" , "Highly infectious, sometimes deadly, disease of cattle" , "could ruin stock market" , "Highly infectious disease of cattle and sheep" , "Disease associated with sheep" .)