Halogen one old woman upset (6)
I believe the answer is:
iodine
'halogen' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'one old woman upset' is the wordplay.
'one' becomes 'i' (roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'woman' becomes 'enid' ().
'upset' says the letters should be written in reverse.
'enid' reversed gives 'dine'.
'i'+'o'+'dine'='IODINE'
Can you help me to learn more?
(Other definitions for iodine that I've seen before include "Hanging tuft" , "Black/violet element" , "Solution applied to wounds as an antiseptic" , "Antiseptic found in seaweed" , "Element that can exist as a purple vapour" .)