Six arrive without their leader ... a nobleman (7)
I believe the answer is:
vicomte
'a nobleman' is the definition.
(I have seen 'French nobleman' mean 'vicomte' so perhaps 'nobleman' could also mean 'vicomte')
'six arrive without their leader' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'arrive' becomes 'come' (coming is a kind of arriving).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'leader' suggests taking the first letters.
The initial letter of 'their' is 't'.
'come' going around 't' is 'comte'.
'vi'+'comte'='VICOMTE'
(Other definitions for vicomte that I've seen before include "Noble" , "French nobleman" , "Foreign nobleman" .)