Glum, working around old tycoon (5)
I believe the answer is:
mogul
'tycoon' is the definition.
(both can mean a rich person)
'glum working around old' is the wordplay.
'working' indicates anagramming the letters.
'around' is an insertion indicator.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'glum' with letters rearranged gives 'mgul'.
'mgul' placed around 'o' is 'MOGUL'.
(Other definitions for mogul that I've seen before include "bigwig" , "Mr Big?" , "Important person - ski slope bump" , "Former Indian ruler; bump on a ski slope" , "big shot" .)