Measure diameter with circles (5)
I believe the answer is:
width
'measure' is the definition.
(width is a kind of measure)
'diameter with circles' is the wordplay.
'diameter' becomes 'd' (mathematics).
'circles' is an insertion indicator.
'd' inserted within 'with' is 'WIDTH'.
(Other definitions for width that I've seen before include "Lateral dimension" , ""Never mind the quality, feel the ___"" , "Breadth" , "Measurement across" , "Broadness" .)