(Answered)) Partial Cover

[danilapal]

The blocking of line of sight works both directions p. 40 (COMMANDER'S EDITION).
If more than one-third (but less than two-thirds) of a ’Mech target is hidden behind blocking terrain, LOS is not considered blocked . Instead, the ’Mech is said to have partial cover p. 41.

Target stay near solid terrain for partial cover (at point-blank range, 0 inch/base-to-base contact to cover terrain).
Attacker in open terrain.

When target will return attack to attacker do attacker have partial cover against target attack?

[nckestrel]

The reciprocal blocking of LOS is for (completely) blocked LOS.  If you can't see them, they can't see you and vice versa.
It has no application on partial cover.  Partial cover is intended to be often applied to only one side and not the other.

Alpha Strike LOS is based on what you actually see at the table.  There is no rule for distance, angles, etc.  If you look from above the attacker to the target, and you see most of the target (more than 2/3rds), there's no cover.  If you look at it's about half covered (you can see more than 1/3 up to 2/3rds), and it's a 'mech, it's got partial cover.  If you see less than 1/3rd of the target, you effectively don't see it (LOS is blocked).
Separately, if the target can't see you, it doesn't matter how much/little you can see of them.  This is primarily to avoid a game breaking exchange between players (i see you but you can't see me at all..).  This isn't a thing.  You can try to get partial cover and them not have partial cover, but you can't claim total cover and still shoot them.