Terribly glum about old magnate (5)
I believe the answer is:
mogul
'magnate' is the definition.
(I've seen this in another clue)
'terribly glum about old' is the wordplay.
'terribly' indicates an anagram.
'about' is an insertion indicator.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'glum' is an anagram of 'mgul'.
'mgul' enclosing 'o' is 'MOGUL'.
(Other definitions for mogul that I've seen before include "Magnate; mound of hard snow on a ski slope" , "Important person; snow mound on a ski slope" , "Powerful person, potentate" , "Mound of hard snow on ski-slope" , "Powerful businessman" .)