Diameter inside accompanying measurement across (5)
I believe the answer is:
width
'measurement across' is the definition.
(I've seen this before)
'diameter inside accompanying' is the wordplay.
'diameter' becomes 'd' (mathematics).
'inside' indicates putting letters inside.
'accompanying' becomes 'with' (I've seen this before).
'd' inserted into 'with' is 'WIDTH'.
(Other definitions for width that I've seen before include "Measurement across" , "Distance from side to side" , "Thickness" , "W in volume HxLxW" , "Lateral measurement" .)