Nef-Crop Entangler

Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.)
You may exert Nef-Crop Entangler as it attacks. When you do, it gets +1/+2 until end of turn. (An exerted creature won't untap during your next untap step.)

Foundations Jumpstart

Amonkhet Remastered

Trample
You may exert Nef-Crop Entangler as it attacks. When you do, it gets +1/+2 until end of turn. (An exerted creature won't untap during your next untap step.)

"If you do not feel the thrill of battle, alive like a flame, you will never triumph."

Amonkhet

All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You can’t do so later in combat, and creatures put onto the battlefield attacking can’t be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.

If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert’s effect preventing it from untapping expires without having done anything.

If you gain control of another player’s creature until end of turn and exert it, it will untap during that player’s untap step.

You can’t exert a creature unless an effect allows you to do so. Similar effects that “tap and freeze” a creature (such as that of Decision Paralysis) don’t exert that creature.