Chaps without leader mark flag (6)
I believe the answer is:
ensign
'flag' is the definition.
(ensign is a kind of flag)
'chaps without leader mark' is the wordplay.
'chaps' becomes 'men' (I've seen this before).
'without leader' means to remove the first letter.
'mark' becomes 'sign' ('sign' can be a synonym of 'mark'**).
'men' with its initial letter removed is 'en'.
'en'+'sign'='ENSIGN'
(Other definitions for ensign that I've seen before include "Ship's colours" , "Badge, flag" , "Commissioned rank in some navies" , "that's flown" , "SECOND PART OF 10" .)