Show anxiety when upset about a species of milfoil (6)
I believe the answer is:
yarrow
'a species of milfoil' is the definition.
(I've seen this before)
'show anxiety when upset about' is the wordplay.
'show' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'anxiety' becomes 'worry' (worry is a kind of anxiety).
'when upset' says the letters should be written in reverse (in a down clue - set, written, up).
'about' means one lot of letters goes inside another.
'worry' back-to-front is 'yrrow'.
'a' going inside 'yrrow' is 'YARROW'.
Can you help me to learn more?
(Other definitions for yarrow that I've seen before include "This herb is strangely a worry" , "Herb; a worry (anag.)" , "Perennial herb, milfoil" , "Creeping plant, also called milfoil" , "Species of milfoil" .)