Let's define to things, first what clipping is, because like many of the terms that we use can and are used to describe different things by different people... like chop block.. and second, what the blocking zone is..
Clipping is a block against an opponent in which the force of the initial contact is from behind and at or below the waist.
The blocking zone is a rectangle centered on the middle lineman of the offensive formation (not necessarily the snapper) and extending five yards laterally and three yards logitudinally in each direction from the line of scrimmage.
Clipping, like blocks below the waist are universally illegal, with exceptions... So, basically the rules say that you cannot clip except in these specific instances.
1) Offensive players who are on the line of scrimmage at the snap within the blocking zone, legally may clip in the blocking zone given the following restrictions.
- A player in the blocking zone may not block an opponent with the force of the initial contact from behind and at or below the knee.
- A player on the line of scrimmage within the blocking zone may not leave the zone and return and legally clip
- The blocking zone disintegrates when the ball leaves the zone
2) When a player turns his back to a potential blocker who has committed himself in intent and direction or movement
3) When a player attempts to reach a runner or legally attempts to recover or catch a fumble, a backward pass, a kick or a touched forward pass, he may push an opponent below the waist at or to the buttocks.
4) When an eligible player behind the neutral zone pushes an opponent below the waist at or to the buttocks to get to a forward pass.
5) Clipping is allowed against the runner.
So, based on your description of the defender blocking the lineman low and from behind you would have a clipping foul, not because of any "retreating" language, because it isn't in the rule, but because clipping in general is illegal except certain times where the offensive team can legally clip and against the runner.