John Gill Commentary 2 Samuel 15:34

"but if thou return to the city, and say unto Absalom, I will be thy servant, O king; as I have been thy father`s servant in time past, so will I now be thy servant; then wilt thou defeat for me the counsel of Ahithophel." — 2 Samuel 15:34 (ASV)

But if you return to the city
To the city of Jerusalem, from where it seems he came:

and say unto Absalom, I will be your servant, O king ;
as he was by usurpation, and by the proclamation of the people with him. David directs him to address him thus, that he might have no suspicion of him, having been an old friend of his:

[as] I [have been] your father's servant hitherto ;
perhaps in the character of a counsellor, as it should seem, since as such he was afterwards employed by Absalom:

so [will] I now also [be] your servant ;
in whatsoever you shall please to employ me under you:

then mayest you for me defeat the counsel of Ahithophel ;
for being taken into Absalom's service, and made one of his counsellors, he would be privy to the advice of Ahithophel, and so be able to work against him.