The crossword clue "helping theorem" typically refers to the word "lemma." A lemma is a subsidiary or minor theorem used primarily to help prove a larger, more important theorem. It is also called a "helping theorem" or an "auxiliary theorem" in mathematics. Summary:

• Lemma: a helping theorem used to prove another theorem.
• Often considered a minor or subsidiary proposition.
• Sometimes lemmas become significant in their own right.

This fits the crossword clue requiring a 5-letter word for "helping theorem".