Rash head of Armenia gets behind old, patriotic song (9)
I believe the answer is:
exanthema
'rash' is the definition.
(I know that rash can be written as exanthema)
'head of armenia gets behind old patriotic song' is the wordplay.
'head of' suggests taking the first letters.
'gets behind' is a charade indicator (letters next to each other).
'old' becomes 'ex' (prefix meaning former or onetime).
'patriotic song' becomes 'anthem' (a national anthem).
The first letter of 'armenia' is 'a'.
'ex'+'anthem'='exanthem'
'a' after 'exanthem' is 'EXANTHEMA'.
(Other definitions for exanthema that I've seen before include "It's rash" , "Eruption" .)