Commander, awfully mad over broken rail (7)
I believe the answer is:
admiral
'commander' is the definition.
(thesaurus)
'awfully mad over broken rail' is the wordplay.
'awfully' indicates an anagram.
'over' says to put letters next to each other.
'broken' indicates anagramming the letters.
'mad' is an anagram of 'adm'.
'rail' anagrammed gives 'iral'.
'adm'+'iral'='ADMIRAL'
(Other definitions for admiral that I've seen before include "Navy rank" , "Naval officer of high rank" , "Naval chief" , "Powerful sailor" , "Supreme naval commander" .)