Solution

STACKING

2 clues

Heaping

Word class: Verb
Nature of clue: Synonym
Difficulty: Very easy
Sources: Boatload Puzzles

Piling up

Word class: Verb
Nature of clue: Description
Difficulty: Very easy
Sources: Boatload Puzzles

Other solutions to Piling up

AMASSING

Words are categorized by artificial intelligence: there can be mistakes.