Glum, working with old magnate (5)
I believe the answer is:
mogul
'magnate' is the definition.
(I know that magnate can be written as mogul)
'glum working with old' is the wordplay.
'working with' indicates anagramming the letters (working with the letters).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'glum' with its letters anagrammed with 'o' can make 'MOGUL'.
(Other definitions for mogul that I've seen before include "Former Indian ruler; bump on a ski slope" , "Mr Big?" , "Big cheese - mound of hard snow on a ski slope" , "Very wealthy or powerful business person" , "Powerful person, potentate" .)