Blocks changes on the radio (6)
I believe the answer is:
altars
'blocks' is the definition.
'changes on the radio' is the wordplay.
'changes' becomes 'alters' (altering is a kind of changing).
'on the radio' indicates a 'sounds like' (homophone) clue.
'alters' is a homophone of 'ALTARS'.
(Other definitions for altars that I've seen before include "Ceremonial tables" , "Tables in Christian churches" , "Church tables" .)