Marksman’s son backtracking about method of authentication (6)
I believe the answer is:
sniper
'marksman's' is the definition.
(sniper is a kind of marksman)
'son backtracking about method of authentication' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'backtracking' says the letters should be written in reverse.
'about' becomes 're' ('re' means regarding or about).
'method of authentication' becomes 'pin' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'pin'='repin'
'repin' reversed gives 'niper'.
's'+'niper'='SNIPER'
Can you help me to learn more?
(Other definitions for sniper that I've seen before include "Concealed sharpshooter" , "He fires shots from hidingplace" , "One shooting from concealed position" , "Critic" , "He shoots from a hidden position" .)