Right-of-centre putsch against Washington — sound of America's extremists? (5)
I believe the answer is:
schwa
'sound of america's extremists?' is the definition.
(vowel sounds of the A's in 'America')
'right-of-centre putsch against washington' is the wordplay.
'right-of-centre' suggests halving the letters (letters on the right of the word).
'against' says to put letters next to each other.
'washington' becomes 'wa' (two letter abbreviation for the state).
'putsch' with half the letters taken is 'sch'.
'sch'+'wa'='SCHWA'
(Other definitions for schwa that I've seen before include "Uh' sound" , "Stress-free sound" , "Unstressed vowel (phonetics)" , "Indistinct sound" , "sound unstressed" .)