Scot mostly takes His Majesty for a fool (5)
I believe the answer is:
schmo
'a fool' is the definition.
(I've seen this before)
'scot mostly takes his majesty' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'takes' indicates putting letters inside.
'his majesty' becomes 'hm' (short for His Majesty).
'scot' with its last letter taken off is 'sco'.
'sco' going around 'hm' is 'SCHMO'.
'for' is the link.
(Other definitions for schmo that I've seen before include "Boring American" , "clot" , "Inept person (US_informal)" , "Fool (Yiddish)" , "jerk" .)