Size slimmer, taking a medium (8)
I believe the answer is:
diameter
'size' is the definition.
(thesaurus)
'slimmer taking a medium' is the wordplay.
'slimmer' becomes 'dieter' (I've seen this before).
'taking' indicates putting letters inside.
'medium' becomes 'm' (as in UK clothing sizes).
'a'+'m'='am'
'dieter' enclosing 'am' is 'DIAMETER'.
(Other definitions for diameter that I've seen before include "Width across the centre of a circle" , "width of bar" , "Paying back (of money)" , "Circle's width" , "Line connecting the centre of a circle with two points on the perimeter" .)