Some mogul’s terribly heavy overcoat (6)
I believe the answer is:
ulster
'heavy overcoat' is the definition.
(I've seen this before)
'some mogul's terribly' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ULSTER' is hidden within 'moguls terribly'.
(Other definitions for ulster that I've seen before include "Man's overcoat" , "Overcoat; N. Ireland" , "Historic Irish province; heavy overcoat" , "Cloak for an Irish province" , "old province" .)