‘But know that YHWH has set apart him who is godly for himself?
YHWH will hear when I call to him.'
Compare Psalms 3:3. He warns his opponents that God has set him apart specially. He is the anointed one, supremely favoured by the covenant God. And he points out that he is still in God's favour, he has been reinstated in godliness. Therefore to rebel against him is to rebel against God. And they must remember that as His anointed one God will hear him when he calls on Him.
Indeed all those who are godly (or those whom God favours) have been chosen by Him for Himself. To touch them is to touch the apple of God's eye (Zechariah 2:8). Thus men should beware of how they treat them.
The word for ‘set apart' also contains the idea of marvellous dealing (see Psalms 17:7; Psalm 39:14). He not only sets them apart but also ‘deals marvellously' with them. It is a dangerous thing to touch YHWH's anointed (Psalms 105:15).
‘Him who is godly.' One who is characterised by covenant love to God, and is within God's covenant love, and therefore ‘one who is favoured by His covenant love'. Therefore they are God's own special possession. That is why He will hear when they call on Him.