An article on Mercury reversing a hot state (5)
I believe the answer is:
ghana
'a hot state' is the definition.
(tropical African country)
'an article on mercury reversing' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'on' says to put letters next to each other.
'mercury' becomes 'Hg' (Hg is the chemical symbol for mercury).
'reversing' is a reversal indicator.
'an'+'a'+'hg'='anahg'
'anahg' back-to-front is 'GHANA'.
(Other definitions for ghana that I've seen before include "African country, W. of Togo" , "Woody raspberry stem" , "West African republic" , "W. African state" , "part of Africa" .)