Circumference, waistline (5)
I believe the answer is:
girth
I believe this is a double definition.
'circumference' is the first definition.
(I know that girth is a type of circumference)
'waistline' is the second definition.
(I've seen this in another clue)
(Other definitions for girth that I've seen before include "Measure all round" , "Distance round the middle" , "Personal circumference" , "Distance around, often of the tummy" , "Perimeter" .)