Person you first have to reckon with (3)
I believe the answer is:
one
'person you' is the definition.
'one' can be an answer for 'you' (I've seen this before). I am not certain of the 'person' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for one that I've seen before include "doesn't count for much" , "Ace" , "Hour after midnight" , "both halves" , "Single, sole" .)