"Half" prefix for circle (4)
I believe the answer is:
semi
'half prefix for circle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for semi that I've seen before include "The last but one round" , "Type of house (UK) - type of truck (US)" , "Property attached to another" , "Not quite the biggest tie" , "Prefix meaning half or partly" .)