A careful eater around morning - for width! (8)
I believe the answer is:
diameter
'for width' is the definition.
(I have seen 'Width of circle ' mean 'diameter' so perhaps 'width' could also mean 'diameter')
'a careful eater around morning' is the wordplay.
'a careful eater' becomes 'dieter' (I've seen this in another clue).
'around' is an insertion indicator.
'morning' becomes 'am'.
'dieter' going around 'am' is 'DIAMETER'.
(Other definitions for diameter that I've seen before include "Paying back (of money)" , "Longest chord of a circle" , "Straight line drawn through centre of circle (8)" , "Two radii" , "Straight line across circle passing through its centre" .)