Rash spreads, quietly acute (5)
I believe the answer is:
sharp
'acute' is the definition.
(I know that acute can be written as sharp)
'rash spreads quietly' is the wordplay.
'spreads' indicates anagramming the letters (I've seen 'spread' mean this).
'quietly' becomes 'p' (p for piano and quiet).
'rash' with letters rearranged gives 'shar'.
'shar'+'p'='SHARP'
(Other definitions for sharp that I've seen before include "Tangy" , "music feature" , "Good cutting edge or acute road turn" , "Clearly defined (image)" , "Liable to cut" .)