He's upset in breaking a leg (5)
I believe the answer is:
nigel
'he's' is the definition.
'upset in breaking a leg' is the wordplay.
'upset' shows that the letters should be reversed in order.
'breaking' indicates an anagram.
'in' reversed gives 'ni'.
'leg' with letters rearranged gives 'gel'.
'ni'+'gel'='NIGEL'
(Other definitions for nigel that I've seen before include "boy" , "-- Kennedy, violinist" , "Man's name" , "__ Mansell, 1992 F1 champion" , "man?" .)