Broke rib on medium edge (4)
I believe the answer is:
brim
'edge' is the definition.
(I know that brim is a type of edge)
'broke rib on medium' is the wordplay.
'broke' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'medium' becomes 'm' (as in UK clothing sizes).
'rib' is an anagram of 'bri'.
'bri'+'m'='BRIM'
(Other definitions for brim that I've seen before include "The edge of a cup or glass" , "Lip of bowl, cup etc" , "Edge or rim" , "Upper edge" , "Lip; part of a hat" .)