Part of a regular curve (3)
I believe the answer is:
arc
'part' is the definition.
(I have seen 'Part of circumference' mean 'arc' so perhaps 'part' could also mean 'arc')
'of a regular curve' is the wordplay.
'of' says the answer is hidden in the clue.
'ARC' is hidden in the letters of 'regular curve'.
(Other definitions for arc that I've seen before include "Curved section" , "Car (anag.)" , "A curve for Joan of . . ." , "Curvature" , "Bow" .)