Rash having primitive song about twitch (9)
I believe the answer is:
urticaria
'rash' is the definition.
(urticaria is a kind of rash)
'primitive song about twitch' is the wordplay.
'primitive' becomes 'ur'.
'song' becomes 'aria' (aria is a kind of song).
'about' means one lot of letters go next to another.
'twitch' becomes 'tic' (tic is a kind of twitch).
'aria' after 'tic' is 'ticaria'.
'ur'+'ticaria'='URTICARIA'
'having' acts as a link.
(Other definitions for urticaria that I've seen before include "Nettle rash" , "Nettlerash" , "complaint" , "hives" .)