Mostly relations of royal men (5)
I believe the answer is:
kings
'men' is the definition.
(I know this)
'mostly relations of royal' is the wordplay.
'mostly' means to remove the last letter.
'relations' becomes 'kins' (kin is a kind of relation).
'of royal' becomes 'gs' (I can't explain this - if you can you should give a lot more credence to this answer).
'kins' with its last letter removed is 'kin'.
'kin'+'gs'='KINGS'
Can you help me to learn more?
(Other definitions for kings that I've seen before include "College etc of royal foundation" , "Two book of the Old Testament" , "Either of two OT books" , "two books in > a number of colleges" , "Name of two Old Testament books" .)