Adam Clarke on Psalms 141:2

As incense - Incense was offered every morning and evening before the Lord, on the golden altar, before the veil of the sanctuary. Exo 29:39, and Num 28:4.

As the evening sacrifice - This was a burnt-offering, accompanied with flour and salt. But it does not appear that David refers to any sacrifice, for he uses not זבח zebach, which is almost universally used for a slaughtered animal; but מנחה minchah, which is generally taken for a gratitude-offering or unbloody sacrifice. The literal translation of the passage is, "Let my prayer be established for incense before thy faces; and the lifting up of my hands for the evening oblation." The psalmist appears to have been at this time at a distance from the sanctuary, and therefore could not perform the Divine worship in the way prescribed by the law. What could he do? Why, as he could not worship according to the letter of the law, he will worship God according to the spirit; then prayer is accepted in the place of incense; and the lifting up of his hands, in gratitude and self-dedication to God, is accepted in the place of the evening minchah or oblation. Who can deplore the necessity that obliged the psalmist to worship God in this way?