Mostly benevolent sergeant-major, a new member of the clan (7)
I believe the answer is:
kinsman
'member of the clan' is the definition.
'mostly benevolent sergeant-major a new' is the wordplay.
'mostly' means to remove the last letter.
'benevolent' becomes 'kind' ('kind' can be similar in meaning to 'benevolent').
'sergeant-major' becomes 'sm' (military abbreviation).
'a new' becomes 'an' (I've seen this in other clues).
'kind' with its final letter taken away is 'kin'.
'kin'+'sm'+'an'='KINSMAN'
(Other definitions for kinsman that I've seen before include "One of the family" , "A blood relation" , "Family member" , "(Male) relative" , "Male relation" .)