Repeat a scarlet circle (4)
I believe the answer is:
redo
'repeat' is the definition.
(I've seen this before)
'a scarlet circle' is the wordplay.
'a scarlet' becomes 'red' (I've seen this before).
'circle' becomes 'O' (O is a circle).
'red'+'o'='REDO'
(Other definitions for redo that I've seen before include "Have second thoughts" , "Redecorate or repeat action" , "Create again" , "Carry out again" , "Decorate (eg a room) again" .)